at PREPOSITION 1) expressing location or arrival. 2) expressing the time when an event takes place. 3) denoting a value, rate, or point on a scale. 4) expressing a state or condition. 5) expressing the object or target of a look, shot, action, or plan. 6) expressing the means by which something is done.
at that — Cf. ↑at that
where it's at — Cf. ↑where it's at
ORIGIN Old English.

English terms dictionary. 2015.