Were
Old English bēon, an irregular and defective verb, whose full conjugation derives from several originally distinct verbs. The forms am and is are from an Indo-European root shared by Latin sum and est . The forms was and were are from an Indo-European root meaning ‘remain’. The forms be and been are from an Indo-European root shared by Latin fui ‘I was’, fio ‘I become’, and Greek phuein ‘bring forth, cause to grow’. The origin of are is uncertain.
wiktionary
From Middle English were, weren, from Old English wǣre, wǣron, wǣren, from Proto-Germanic *wēz-, from Proto-Indo-European *h₂wes-. More at was.
From Middle English were, wer, see wer.
Back-formation from werewolf and other terms in were-, from the same source as English wer, were(“man”) (above).
etymonline
were (v.)
Old English wæron (past plural indicative of wesan) and wære (second person singular past indicative); see was. The forms illustrate Verner's Law (named for Danish linguist Karl Verner, 1875), which predicts the "s" to "z" sound shift, and rhotacism, which changed "z" to "r." Wast (second person singular) was formed 1500s on analogy of be/beest, displacing were. An intermediate form, wert, was used in literature 17c.-18c., before were reclaimed the job.
