Old English æt , from Proto-Germanic *at (cf. Old Norse, Gothic at , Old Frisian et , Old High German az ), from PIE *ad- "to, near, at" (cf. Latin ad "to, toward" Sanskrit adhi "near;" see ad- ).
Lost in German and Dutch, which use their equivalent of to ; in Scandinavian, however, to has been lost and at fills its place. In choosing between at church , in church , etc. at is properly distinguished from in or on by involving some practical connection; a worshipper is at church; a tourist is in the church.
The colloquial use of at after where ("where it's at") is attested from 1859. At last is recorded from late 13c.; adverbial phrase at least was in use by 1775. At in Middle English was used freely with prepositions (. at after , which is in Shakespeare), but this has faded with the exception of at about , which was used in modern times by Trollope, Virginia Woolfe, . Lawrence, and Evelyn Waugh, but nonetheless is regarded as a sign of incompetent writing by my copy editor bosses.