Knickers are hardly a new idea. But not since the Twenties and Thirties, when young boys wore corduroy knickerbockers, have they been so popular.

Believed to be named after Diedrich Knickerbocker (fictitious author of Washington Irving's "History of New York"), the popular dress of Dutch burghers and newsboys has become the latest edition for many young women.

A favorite of Paris designers--particularly Yves Saint Laurent, who has shown velvet and satin evening versions for years--knickers in their current mode are meant for day as well. Even for the office.

When designers started fiddling with pants shapes, softening the structured, man-tailored styles with unpressed pleats, soft fabrics and fullness, knickers became part of the assortment (along with culottes, bloomers, harem pants, whatever). Kids in London last spring--and soon after, in New York--took to the streets in Regency dress, particularly knickers and ruffled shirts.

"For most women it makes the leg look terrific," says Val Cook, Saks-Jandel fashion director.

That selling point may be hard to resist. And maybe that's why knickers were one of the first things to sell when fall clothes first hit the stores.

You can expect to see knickers worn by a lot of women--some of whom shouldn't--this fall and winter. For events billed as very dressy there will be velvet knickers with fussy blouses, satin knickers and velvet jackets, or any combination of dressy fabric and ruffles.

For office wear, watch for corduroy, wool, tweed or plaid knickers with short jackets or soft sweaters. Or as a preppie spinoff, with cable-stitch pullover, oxford shirt and even a little bow tie.

And for casual, comfortable weekends, we're predicting a lot of people--men and women--looking like cross-country skiers in cord knickers, bulky sweater, sturdy shoes and ski socks.