step-in

alushousut puhekieltä An item of clothing which one steps into to put on; specifically, women's panties.
1931, (w), Sanctuary, Vintage 1993, p. 46:

‘Doc got that step-in in Memphis,’ the third said. ‘Off a damn whore.’

One of the girls hoisted her skirt suddenly, pulled and ripped at her pink step-ins and tore them to a sizeable flag; then, screaming ‘Ben! Ben!’ she waved it wildly.