Green’s Dictionary of Slang

hard-on adj.

[hard-on n.]

1. sexually aroused.

[US]C. Cooper Jr Weed (1998) 127: Fresh home and hard-on, he’d found the street and apartment.

2. tough, aggressive.

[US]Kerouac letter 21 July in Charters II (1999) 55: Don’t be bugged by what Cowley wanted put in [...] about ‘intellectualism’ of you & Bill & Joan as against Neal’s hard-on hungry purity.
[US]D. Woodrell Muscle for the Wing 98: Tell your hard-on brother what you seen over at Frechette Park.