Green’s Dictionary of Slang

slashing adv.

[slashing adj.]

excellently, wonderfully, exceptionally.

[UK]Swell’s Night Guide 76: Don’t he chant that’ere slashing? O, rumbo!
[UK]Farmer & Henley Sl. and Its Analogues.
[US]Nat. Police Gaz. (NY) 5 May 6/2: [headline] [of fighting cocks] three slashing good chickens.