Green’s Dictionary of Slang

knock back v.

1. [20C+] to cost, e.g. that’ll knock you back a bit.

2. [1910s+] to fine someone.

3. [1910s+] (also bang back, knock about) to eat, to drink, esp. to finish off one’s drink.

4. [1930s+] (orig. Aus./N.Z.) to reject [knockback n. (1)].