So, let's talk about the suit first. It's fine wool, blue-grey, double-breasted pinstripe, no vents. Beautiful cut. Cost a lot of dollars. Maybe not telephone numbers, but decidedly expensive. Small inch of white cuff, folded white handkerchief in breast pocket. A glimpse of blue braces. Shoes are brown brogues, polished. The tie is a challenge, balancing on the edge of flash but holding it there. The hair - neither long nor short - is topped by a few boyish curls that'd pull at your heart.

And here's something else which only I know, though I'll share it with you if you promise not to tell: underneath it all, this cool dude is wearing white Y-fronts.

How do I know? Because I saw them laid out in her dressing room, ready to wear. Her? Yes, the man in the suit, swanking it round the stage, sexy eyes prowling the audience, shoulders giving the Manhattan shrug, shoes soft-shuffling to Screamin' Jay Hawkins, is none other than - put your hands together, please, ladies and gentlemen for - Peggy Shaw, gender icon of the Lower East Side, a woman passing as a man in her current show in London called Menopausal Gentleman.

Shaw, a tall, lean Bostonian, 53 (though what's that nowadays?), with a soft kiss and a strong handshake, is just coming to the end of the hot flushes, the drying up, the angst, the terror of whatever's on the other side of the hill. Hers was a tough menopause and one of her coping strategies is to dress up in a smart suit and tell everyone about it, using a disconcerting mix of wit, humour and pain.

During the one-hour show, she laughs about the uncomfortable night sweats, taking out the neatly-folded handkerchief to mop her brow. Later, she describes the fiery passion she still feels about living and loving, rails against the threat of stiff bones ("Dem bones, dem bones, dem dry bones") paces the stage like the beast she feels is within her, dances in the dark to Nina Simone and moves through the audience shaking hands as she talks through her own version of I Did it My Way so that at first you laugh at the send-up, at the kitsch sincerity of it until, listening to the words, you realise she's talking about herself. It's then you hear the loneliness and the heartbreak, see the vulnerability beneath the swagger.

"It's hard to be a gentleman," she tells us, coyly crossing her hands in front of the family jewels. But doesn't say how hard it was to be lesbian 30 years ago in an Irish family in Boston.

"And lonely," Shaw told me in her dressing room. "I was the middle of seven children and different. I felt like a freak of nature." Her grandfather was from Northern Ireland. So, did a Catholic upbringing take its toll? "No, but the Protestant one did." The family was poor, her mother had a succession of breakdowns, the children were beaten with a stick when bad. She was 17 when her father died at the age of 47. Shortly after that, she got all dolled up to go to a friend's wedding.

"My mother, who was watching me dress up in my suit, sighed and said I looked just like my father when they first met. I could feel her starting to fall in love again with him in me. She wanted me to look like him, to be like him."

It gave her the idea for her performance You're Just Like My Father, the show in which she examines the roots of her sexuality. "It started off as a performance about my father. I really loved his clothes, the way they were laid out, the shirts folded, the smell of his things. All that." She gestures and my eyes fall on the Y-fronts, the tie, a pair of gold cuff links laid out for the night's performance. "But as I was working on it, it turned out to be all about my mother and what she wanted me to become."

Shaw moved to New York and fell in love with a woman. When the woman left her, she turned to her best friend - and married him. Lost a best friend and gained a husband, I suggest, but she shakes her head. It wasn't like that. Though when they broke up - they had a two-year-old daughter, Shara, by then - he didn't want to know.

Shaw started devising her own shows, touring Europe, in the 1970s with a group of drag queens, living in a London squat with Shara, pinching food from local shops. Round about then, she met up with Lois Weaver, with whom she lived and worked for 20 years. They did lots of shows together, though last year, in New York, on her own, Shaw created the role of Billy Tipton, the 1950s jazz musician who amazed everyone - including his wives - by turning out to be a woman.

But why the suit for a performance about the menopause? "I'm attracted to suits," she says. "I always feel comfortable in them. My feminism comes through my clothes. The suit is safe. It protects me. When I put it on, I know I can be anybody."

Passing as a man also allows her a little extra time. On-stage, she tells us: "I'm a 53-year-old woman who passes as a 35-year-old man. A woman passing as a man looks like a younger man. A man passing as a woman looks like an older woman. That's just the way it goes. It's a trade-off. I sacrifice being a woman for youth." And she smiles knowingly at the audience, a cat who's licked the cream. For once, age is on the side of the older woman. I wonder about using the F word and decide to risk it. Does she ever wear a frock? I ask. "Only when I have to. On stage."

The break with Lois Weaver a few years ago was hard. They'd lived and worked together for so long that they no longer had separate identities. "We had to wrench ourselves apart. Lois had helped me bring up Shara. We'd formed the Split Britches Theatre Company together." They're finally apart now, though they still share a mountain cabin where they write the material for their shows. Next month, they'll be doing a performance together in the La Mama Theatre in New York. "We redefine our relationship the whole time." She pauses. "It's been a long, hard road to get here," she adds, her voice leaning on a blue note.

But not for long. On-stage, she's a lion who roars, who wants it all, still. She has already started writing the show that is the next part of her life - all about her four-year-old grandson and how to raise him as a boy. There are problems out there, she knows, but as in the past, she'll do it her way. Roaring. In a sharp suit. This is not a woman who intends to go gentle.

Split Britches, an anthology of play- texts by Peggy Shaw, is published by Routledge, price £14.99 in UK. Available direct from 0044 8700 768853.