What's the "een" in Halloween?
1660s, from Latin hibernationem (nominative hibernatio) "the action of passing the winter," noun of action from past participle stem of hibernare "to winter, pass the winter, occupy winter quarters;" related to hiems "winter," from PIE *gheim- "snow, winter" (cf. Sanskrit heman "in winter," Hittite gimmanza, Greek kheima, Old Church Slavonic zima, Lithuanian žiema "winter").
An inactive state resembling deep sleep in which certain animals living in cold climates pass the winter. In hibernation, the body temperature is lowered and breathing and heart rates slow down. Hibernation protects the animal from cold and reduces the need for food during the season when food is scarce. Compare estivation.
Passing the winter in a sleeping or inactive condition. Bears, ground squirrels, woodchucks, and several other kinds of animals hibernate.