Green’s Dictionary of Slang

1. to create, to make.

1906		E. Dyson Fact’ry ’Ands 234: It was spatterin’ ’ot paste ez if it was being’ whacked out by er maxim.
1915		Wodehouse Psmith Journalist (1993) 180: If a certain amount of harmless revelry can be whacked out of Fourth Avenue, we must dash there with the vim of highly-trained smell-dogs.
1934		Wodehouse Right Ho, Jeeves 71: When one has taken the trouble to whack out a highly juicy scheme.
1983		A. Payne ‘Willesden Suite’ Minder [TV script] 41: I got to whack out seven hundred melon balls.

2. to play (music), to ‘knock out’ (a tune).

1915		Wodehouse Psmith Journalist (1993) 218: Two fiddles and a piano whacking out ‘Beautiful Eyes.’.
1923		Wodehouse Inimitable Jeeves 91: The chappie at the piano whacked out a well-meant bar or two.

3. see whack v.¹ (2)

4. see whack v.¹ (6)

5. see whack v.² (1)