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Related to at it: Have at it
1. Involved in an action, activity, or pursuit vigorously, exhaustively, or determinedly. I studied really hard for this test; I was at it all night. I wish those dogs would quit barking, they've been at it since the sun came up!
2. slang Engaging in sexual intercourse. I can always hear when the neighbors are at it. It's really embarrassing!
Vigorously pursuing an activity, especially a fight, but also sex or some other activity. For example, Whenever they play bridge they really go at it (fight), or The new job keeps Tom at it day and night (works hard), or In the spring the dogs are always at it (sex). Shakespeare used this seemingly modern idiom for "fighting" in Troilus and Cressida (5:3): "They are at it, hark!" [Late 1500s]
at itengaged in some activity, typically a reprehensible one.
1993 G. F. Newman Law & Order Oh, don't take me for a complete idiot, Jack. I know you're at it.
Engaged in verbal or physical conflict; arguing or fighting: The neighbors are at it again.