Green’s Dictionary of Slang

a baseball bat.

1998		K. Sampson Awaydays 36: I’ve been going away with Tranmere for over two years now and I’ve never shit. Even when Stockport surrounded us [...] and came at us with basies.
2001		N. Griffiths Sheepshagger 11: Fuckin Ianto should’ve let us know they were coming. Would’ve been up yer with a fuckin basie I would’ve aye.