"middle part of the body," also "part of a garment fitted for the waist," late 14c., probably from Old English *wæst "growth," hence, "where the body grows," from Proto-Germanic *wahs-tu- (cf. Old English wæstm, Old Norse vaxtr, 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- "to increase" (see augment). Meaning "portion of a garment that covers the waist" (but, due to fashion styles, often is above or below it) is from 1640s.
waist (wāst)
n.
The part of the human trunk between the bottom of the rib cage and the pelvis.
The middle section or part of an object, especially when narrower than the rest.