Green’s Dictionary of Slang

1. (US) to leave.

1953

S. Bellow Augie March (1996) 273: The old twelvemonth sagging off with his scythe and Diogenes lantern.

2. (UK juv.) to truant from school.

1975		A. Bleasdale Scully 43: We often go up there when we’re sagging school.
1997		B. Morrison As If (1998) 36: The boys were ‘sagging’, truanting.
2001		OnLine Dict. of Playground Sl. 🌐 sag off n. bunk off school, i.e. be absent without permission. Kids sagging off often said to be suffering from ‘saggeritis disease’.
2003		N. Griffiths Stump 50: What does he expect me to do? Lerrim go cos we sagged a few classes together.