Old English man, mann "human being, person (male or female); brave man, hero; servant, vassal," from Proto-Germanic *manwaz (cf. Old Saxon, Swedish, Dutch, Old High German man, German Mann, Old Norse maðr, Danish mand, Gothic manna "man"), from PIE root *man- (1) "man" (cf. Sanskrit manuh, Avestan manu-, Old Church Slavonic mozi, Russian muzh "man, male").
Plural men (German Männer) shows effects of i-mutation. Sometimes connected to root *men- "to think" (see mind), which would make the ground sense of man "one who has intelligence," but not all linguists accept this. Liberman, for instance, writes, "Most probably man 'human being' is a secularized divine name" from Mannus [cf. Tacitus, "Germania," chap. 2], "believed to be the progenitor of the human race."
Sense of "adult male" is late (c.1000); Old English used wer and wif to distinguish the sexes, but wer began to disappear late 13c. and was replaced by man. Universal sense of the word remains in mankind and manslaughter. Similarly, Latin had homo "human being" and vir "adult male human being," but they merged in Vulgar Latin, with homo extended to both senses. A like evolution took place in Slavic languages, and in some of them the word has narrowed to mean "husband." PIE had two stems: *uiHro "freeman" (cf. Sanskrit vira-, Lithuanian vyras, Latin vir, Old Irish fer, Gothic wair) and *hner "man," a title more of honor than *uiHro (cf. Sanskrit nar-, Armenian ayr, Welsh ner, Greek aner).
Man also was in Old English as an indefinite pronoun, "one, people, they." The chess pieces so called from c.1400. As an interjection of surprise or emphasis, first recorded c.1400, but especially popular from early 20c. Man-about-town is from 1734; the Man "the boss" is from 1918. To be man or mouse "be brave or be timid" is from 1540s. Men's Liberation first attested 1970.