The story
Mimics the form of C error codes: for example, on Linux, a library call may return the error code ENOMEM to indicate that it has run out of memory. TUITS comes from round tuit.
Etymology from Wiktionary, CC BY-SA 4.0.
ENOTUITS
Today it means: A fictitious error code indicating that a person has run out of round tuits (i.e. they don't have any time to spare on the task being discussed). Full definition →