MEN'S DRESS in Eastern America, after several hundred years of drably stable masculinity, is undergoing an ironic feminization. Ironic because the source of the femininity is that quintessential American male -- the cowboy.

Madison Avenue would have us visualize the cowboy as the apotheosis of masculinity, the indomitable outdoorsman, rugged wrangler, macho Marlboro man.

But look more closely. Look at what the Marlboro man and now his millions of eastern imitators are wearing today: boots, for gracious sakes, with high heels and little pointy toes, curl brim hats festooned with featers like a Victorian matron's; daintily tooled leather belts; shirts with cutesy-pootsie smile pockets; and, of course, jewelry.

Fast disappearing is the unadorned granite rectangularity of traditional eastern male dress. True, the gray flannel suit and white shirt, the durable uniform of businessmen for so many generatios, began giving way here and there as far back as the 1960s to leisure suits, double knits, Nehru jackets and other new phenomena. Many of these fashions were short-lived, however, and most still adhered to the basic masculine form. Now they are all surrendering to modern-day cowboy regalia -- not the simple unstudied, coarse-leather original trappings of yore, but a sissified 20th century haute cuture variant of swirling curves and colors.

No level-headed eastern man brought up by the back of his mama's hand in a more sensible area would have been seen dead or alive in the daylight in such frippery. But today, western costume has invaded the sanctum of eastern manhood, reducing stolid male attire to frivolous weekend playclothes and turning the masculine sartorial sense to simpering feminine mush.

Starting in New York, it is rapidly reaching out toward every eastern man -- from the Wall Street broker and the Harlem street hustler to the Maine lobsterman and the last frecklebelly in south Georgia. Even the president of the United States, admittedly a victim of prlonged western exposure, wears high heels on occasion. Media pictures of him lounging in hand-tooled leather cowboy boots abound in the land.

Western customery is also big business in the East. Stores sold more than $500 million in cowboy hats alone last year. Designer jeans are a ubiquitous and indiscriminate staple of secretaries and steamshovel operators alike. The styles, packaging and sales pitch for men, in fact, are largely indistinguishable from those for women.

There is even a Cowboy Catalog available in book stores -- 192 picture-packed pages of cowboy hats, cowboy shirts, cowboy chaps, cowboy buckles, cowboy jewelry, even whole cowboy suits. Yes, cowboy suits for full-grown men. These are not your rough-and-ready getups for the long cattle drive across Wyoming, but sleek Oleg Cassini costumes and verticle stitch patterns for business occasions and matching gaudy trousers and jackets encrusted with rhinestones for playtime.

But that's only the beginning. The Cowboy Catalog, written by Sandra Kauffman and published by Clarkson N. Potter Inc. in New York (where else?), has a boot section that reads like a 1910 Sears Roebuck ladies' footwear guide: boots with kitty-tan leather shafts and lizard vamps, boots with needle-point toes, boots with underslung high heels, boots that are scallop-topped, boots with leather inlays and overlays embossed with petitie flowers, birds and butterflies.

Butterflies? On men's boots? What will the U.S. Cavalry say?

Then there are the hats: great curving headdresses of rabbit, nutria and beaver, some finished in angora and bedecked with braid, beads, feathers aned even ostrich plumes -- something your great-grandmother might have worn to an afternoon tea.

There's more. Shirts: baby-lamb suede blouses embroidered in satin, complete with smile pockets, matching plaid yoke and cuffs, bib front and dainty stitchery. Jeans: flare-bottom Le Poche originals with Rolls-Royce monograms in gold lurex. Buckles: heart-shaped belt clasps engraved with rosebuds and huge filigreed buckles set with rubies. Belts: fine-grain leather embossed with acorns, flowers and fruit. Accessories: pink bandannas, bolo ties with 14-karat gold overlaid slides, gold-plated collar tips and silver boot ornaments.

And if you really take this business seriously, you can get a pair of bat-wing chaps, which were originally designed to protect the rider against the thick brush of the range but which in the late 20th century resemble little more than vestigial bloomers.

The incursions of western wear into the male East are growing with each day. There seems to be little hope of a reversal. So beware. The man next to you may be a closet cowboy.