"structure built by a bird or domestic fowl for the insulation and rearing of its young," Old English nest "bird's nest; snug retreat," also "young bird, brood," from Proto-Germanic *nistaz (source also of Middle Low German, Middle Dutch nest, German Nest; not found in Scandinavian or Gothic), from PIE *nizdo- (source also of Sanskrit nidah "resting place, nest," Latin nidus "nest," Old Church Slavonic gnezdo, Old Irish net, Welsh nyth, Breton nez "nest"), probably from *ni "down" + from PIE root *sed- (1) "to sit."
From c. 1200 of an animal or insect. Used since Middle English in reference to various accumulations of things, especially of diminishing sizes, each fitting within the next (such as a nest of drawers, early 18c.). Nest egg "retirement savings" is from 1700; it was originally "a real or artificial egg left in a nest to induce the hen to go on laying there" (nest ei, early 14c.), hence "something laid up as the beginning of a continued growth."