Definition of "at it"

Mart. Oh they are at it.Lart. Their noise be our instruction.

c. 1608–1609 (date written), William Shakespeare, “The Tragedy of Coriolanus”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, (please specify the act number in uppercase Roman numerals, and the scene number in lowercase Roman numerals)

We were at it until three o'clock in the morning.

1893, Edwin H Porter, The Fall River Tragedy

He's always at itYou're always at it'Cause you're an orgasm addictWe're all orgasm addicts

1977, Buzzcocks (lyrics and music), “Orgasm Addict”