The word 'man' has been in the English language since the beginnings of the language. It originally meant a human being, irrespective of sex, and in Old English there were two words for a male human being: were and wapman. Both these words became obsolete after the 13th century. The only vestige of the former in modern English is the word werewolf. Were here is a cognate of Latin vir meaning man (cf virile) and of the earlier Sanskrit vira, also meaning man.
Many people still insist that man means 'a person of either sex', although they would be horrified if I or other women went through the public toilet door marked 'Men'. If I had started the last sentence "many men still insist that ..." few, if any, readers would have understood this to refer to both men and women.
The OED now specifically states (and I quote): "The genderless uses of man to mean ‘human being’ or ‘person’ are now often objected to on the grounds that they depreciate women, and are frequently replaced by human, human being, or person."
Were may have disappeared in modern English, but the opposite of were in Old English hasn't. That was wif, which originally meant any woman, regardless of marital status. This meaning is still retained in words and phrases such as fishwife and old wives' tales. The modern word 'woman' was originally wifman, which was literally 'woman human being'. W is pronounced with rounded lips, but i is pronounced with the mouth and lips spread wide, so by the end of the 12th century the initial rounding of the lips influenced the pronunciation of the following vowel, causing it to change from an i to u, the spelling later changing to the o we have today.