Green’s Dictionary of Slang

k.b. v.

[abbr. knock back v. (4)]

to reject.

[Scot]I. Welsh Trainspotting 76: After k.b.ing me for ages, she’d eventually let me shag her.
[Scot](con. 1980s) I. Welsh Skagboys 210: Esther realises that even he’s KB’d her.