Green’s Dictionary of Slang

(orig. US) sexually excited.

1947		K. Amis letter 20 Mar. in Leader (2000) 120: He says ‘the thought of her kept me warm a whole stage’. Horned-up little bastard.
1967–8		Baker et al. CUSS.
1974		C. Loken Come Monday Morning 26: He remembered seein’ ’em [...] on the Farm so horned up they’d try to mount the stallion they was turned in with.
1998		I. Welsh Filth 205: Even the short drive gets me horned up again.