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 (e.g. 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, D.H. Lawrence, and Evelyn Waugh, but nonetheless is regarded as a sign of incompetent writing by my copy editor bosses.
Old English hwær, hwar, from Proto-Germanic *khwar (cf. Old Saxon hwar, Old Norse hvar, Old Frisian hwer, Middle Dutch waer, Old High German hwar, German wo, Gothic hvar "where"), from PIE interrogative base *qwo- (see who).
The symbol for the element astatine.
The symbol for astatine.
At the site of stimulating and modish events, trends, etc; where the action is: Where the important stuff is going on. This is where it's at/ Why should only book writers write books? They're not where it's at/ TV is where it's at
The essential locus of the truth; the core of things: A lot of cats are finding out where it's at in the joint (1960+)