Green’s Dictionary of Slang

but only! excl.

(S.Afr.) an excl. used to support a statement made by one’s companion.

[SA]CyberBraai Lex. at [Internet] BUT ONLY: A very useful phrase now going out of fashion, it falls into the same category as the American ‘as if . . .’ It is used to give support to a statement by your companion.