At It Meaning

Example: Used other than as an idiom: see at,‎ it.
Occupied with a given activity; busy with something. [from 17th c.]
c. 1608, William Shakespeare, Coriolanus, First Folio 1623:
  Mart. Oh they are at it.
  Lart. Their noise be our instruction.
1893, Edwin H Porter, The Fall River Tragedy:
  We were at it until three o'clock in the morning.