Green’s Dictionary of Slang

to defeat utterly, to thrash, to criticize harshly.

1920		Ade Hand-made Fables 106: It is not necessary to travel 4,000 miles in order to loaf in front of Shop Windows or slather the Express Orders on Junk.
2003		McGill Reed Dict. of N.Z. Sl. 191: slather To scold [...] ANZ from c.1920.