Green’s Dictionary of Slang

blow off n.3

[SE blow, to blow something away with a puff of air]

1. (US campus) a lazy person, a ‘layabout’.

[US]Baker et al. CUSS 84: Blows (it) off A person who always fools around.

2. (US teen) anything considered exceptionally easy; also as adj.

[US]J. Doyle College Sl. Dict. 🌐 blow off [Purdue] [...] (2) something very easy.
[US]D. Burke Street Talk 2 13: a blow-off class – an easy class.