first person singular present indicative of be (q.v.); Old English eom "to be, to remain," (Mercian eam, Northumbrian am), from Proto-Germanic *izm(i)-, from PIE *esmi- (source also of Old Norse emi, Gothic im, Hittite esmi, Old Church Slavonic jesmi, Lithuanian esmi), first person singular form of root *es- "to be."
In Old English it formed only present tenses, all other forms being expressed in the W-BASE (see were, was). This cooperative verb is sometimes referred to by linguists as *es-*wes-. Until the distinction broke down 13c., *es-*wes- tended to express "existence," with beon meaning something closer to "come to be."
Old English am had two plural forms: 1. sind/sindon, sie and 2. earon/aron. The s- form (also used in the subjunctive) fell from English in the early 13c. (though its cousin continues in German sind, the 3rd person plural of "to be") and was replaced by forms of be, but aron (see are) continued, and as am and be merged it encroached on some uses that previously had belonged to be. By the early 1500s it had established its place in standard English.