Green’s Dictionary of Slang

knock-down n.

1. in senses of lit. or fig. violence or aggression.

(a) [mid-17C] something astounding, remarkable, that ‘knocks one down’.

(b) [late 17C–19C] (also knocker-down) strong ale or liquor.

(c) [19C+] (US) a fight; also in fig. use, an undeniable argument.

2. [mid-19C+] (Aus./N.Z./US) an introduction, esp. a formal introduction of a man to a woman in whom he is interested; thus give one a knock-down, to give one an introduction.

3. [1930s–60s] (US) information.

4. [1950s] (Aus.) in financial/commercial uses [? abbr. knock-down money].

(a) a profit, usu. illicit.

(b) a discount.

In compounds

knock-down money (n.)

[mid-19C; 1980s+] (US) tips or gratuities.

In phrases

give a knockdown (to) (v.)

[late 19C] (US) to recognise.

take a knock-down (v.)

[1950s] (US black) to take note.