Old English losian "be lost, perish," from los "destruction, loss," from Proto-Germanic *lausa- (cf. Old Norse los "the breaking up of an army;" Old English forleosan "to lose, destroy," Old Frisian forliasa, Old Saxon farliosan, Middle Dutch verliesen, Old High German firliosan, German verlieren), from PIE root *leu- "to loosen, divide, cut apart, untie, separate" (cf. Sanskrit lunati "cuts, cuts off," lavitram "sickle;" Greek lyein "to loosen, untie, slacken," lysus "a loosening;" Latin luere "to loose, release, atone for, expiate").
Replaced related leosan (a class II strong verb whose past participle loren survives in forlorn and lovelorn), from Proto-Germanic *leusanan (cf. Old High German virliosan, German verlieren, Old Frisian urliasa, Gothic fraliusan "to lose").
Transitive sense of "to part with accidentally" is from c.1200. Meaning "fail to maintain" is from mid-15c. Meaning "to be defeated" (in a game, etc.) is from 1530s. Meaning "to cause (someone) to lose his way" is from 1640s. To lose (one's) mind "become insane" is attested from c.1500. To lose out "fail" is 1858, American English. Related: Lost; losing.
To become very distraught; lose control of oneself; suffer a kind of breakdown; crack up: told me to keep an eye on Dennis, that he looked like he was losing it/ of being totally stressed out, of ''losing it''/ Thieu was really tough, but that day he lost it. His spirit was broken
[1990s+; probably fr the notion of losing one's grip on reality]