- \ \ [OE] There are four distinct components that go to make up the modern English verb be. The infinitive form be comes ultimately from an Indo-European base *bheu-, *bhu-, which also produced, by other routes, future and physical.\ \ Its Germanic descendant was *bu-, which signified on the one hand ‘dwell’ (from which we get booth, bower, byre, build, burly, byelaw, and the final element of neighbour), and on the other hand ‘grow, become’, which led to its adoption as part of the verb expressing ‘being’ (in Old English particularly with the future sense of ‘coming to be’). Am and is go back to the ancient Indo- European verb ‘be’, *es- or *s-, which has contributed massively to ‘be’ verbs throughout all Indo-European languages (third person present singulars Greek esti, Latin est, French est, German ist, Sanskrit ásti, Welsh ys, for example) The Indo-European first and third person singular forms were, respectively, ésmi and ésti.\ \ For the present plural Old English used the related sind(on) (as found in Latin sunt, French sont, and German sind), but this died out in the 12th century, to be replaced by are, which comes from a Germanic base *ar- of unknown origin.\ \ From the same source is the now archaic second person singular art.\ \ The past tense forms was, were come ultimately from an Indo-European base *weswhich meant ‘dwell, remain’. Related words in other Indo-European languages include Sanskrit vásati ‘dwell, remain’ and Gothic wisan ‘remain, continue’.\ \ Cf.⇒ BOOTH, BOWER, BUILD, BURLY, BYELAW, BYRE
Word origins - 2ed. J. Ayto. 2005.