1. hard at work exerting oneself:
- He never gives himself any rest; he’s always at it.
2. doing smth., especially repeatedly, of which the speaker disapproves:
- Those children lighting fires in the park. So they are at it again, are they?
3. arguing or quarrelling, especially when this happens repeatedly:
- There they are at it now, with their caterwauling, squealing, all together.
4. (euph., sl.) indulging in sexual intercourse:
- The couple next door have been at it all night.
in it—taking part in an activity; engaged in smth.:
- There is no money in boxing. Those of us who started it many years ago are really not in it for the money.