What is the X in X-mas?
1770, "severe homesickness" (considered as a disease), Modern Latin (cf. French nostalgie, 1802), coined 1668 by Johannes Hofer, as a rendering of German heimweh, from Greek algos "pain, grief, distress" (see -algia) + nostos "homecoming," from PIE *nes- "to return safely home" (cf. Old Norse nest "food for a journey," Sanskrit nasate "approaches, joins," German genesen "to recover," Gothic ganisan "to heal," Old English genesen "to recover"). Transferred sense (the main modern one) of "wistful yearning for the past" first recorded 1920.
nostalgia nos·tal·gia (nŏ-stāl'jə, nə-)
A bittersweet longing for things, persons, or situations of the past.
The condition of being homesick; homesickness.