What is the X in X-mas?
Old English nest "bird's nest, snug retreat," also "young bird, brood," from Proto-Germanic *nistaz (cf. Middle Low German, Middle Dutch nest, German Nest), from PIE *nizdo- (cf. Sanskrit nidah "resting place, nest," Latin nidus "nest," Old Church Slavonic gnezdo, Old Irish net, Welsh nyth, Breton nez "nest"), probably from *ni "down" + *sed- (1) "to sit" (see sedentary).
Used since Middle English in reference to various accumulations of things (e.g. a nest of drawers, early 18c.). Nest egg "retirement savings" is from 1700, originally "a real or artificial egg left in a nest to induce the hen to go on laying there" (c.1600).