Wiktionary
step-in
n. (context also in plural English) An item of clothing which one steps into to put on; specifically, women's panties.
WordNet
Usage examples of "step-in".
She tottered after several aflight and erratic cards, stepping out of her step-in mules.
He sat atop a footstool, engaged in double-lacing his shoes, taking time between knots to watch her dress, an operation that seemed to portray the correspondence between position and time, one action generating the next, step-in, shake-into, hoist-on, her limbs and torso covered now, fluidly moving woman, her eyes appearing to follow the delicate pebbling sound of Softly's voice.