at it

• (Received Pronunciation) IPA^(key): /ˈat ɪt/
• (General American) IPA^(key): /ˈæt ɪt/, [ˈæɾɪt̚]

at it

1. Occupied with a given activity; busy with something. [from 17th c.]
   • 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, →OCLC, (please specify the act number in uppercase Roman numerals, and the scene number in lowercase Roman numerals):

     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.

2. Engaged in sexual intercourse or other sexual activity.

   I came home early and caught them at it in the front room.

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

     He's always at it
     You're always at it
     'Cause you're an orgasm addict
     We're all orgasm addicts

• go at it
• while one is at it

• Tait, Tati, tait