late 14c., "middle part of the body," also "part of a garment fitted for the waist, portion of a garment that covers the waist" (but, due to fashion styles, often above or below it), probably from Old English *wæst "growth," hence, "where the body grows," from Proto-Germanic *wahs-tu- (source also of Old English wæstm, Old Norse vöxtr, Swedish växt, Old High German wahst "growth, increase," Gothic wahstus "stature," Old English weaxan "to grow" see wax (v.)), from PIE *wegs-, extended form of root *aug- (1) "to increase."
<a href="https://www.etymonline.com/word/panty-waist">Etymology of panty-waist by etymonline</a>
Harper, D. (n.d.). Etymology of panty-waist. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/panty-waist