Green’s Dictionary of Slang

rap v.1

1. in intransitive senses pertaining to speech.

(a) [late 16C+] to curse.

(b) [mid-18C–mid-19C] to swear a false oath, to perjure oneself; thus as n. a false oath.

(c) [mid-19C] (UK Und.) to stand accused, to appear guilty.

(d) [mid-19C+] to inform, esp. to the police.

(e) [late 19C+] to talk, to converse.

(f) [1960s+] to have any form of impromptu dialogue.

(g) [1960s+] (US black) to speak lines in the dozens n. (1)

2. in transitive senses pertaining to speech.

(a) [mid-18C–mid-19C] to swear (evidence) against someone; or for someone.

(b) [late 19C+] to attack verbally, to criticize, to say sharply.

(c) [late 19C-1920s] (US Und.) to recognize.

(d) [20C+] (US) to charge; to prosecute, to arrest with a view to prosecution.

(e) [1940s] (US) to persuade, to trick out of.

(f) [1950s+] (Aus., also rap up) to praise, esp. to praise to excess.

(g) [1960s+] to indulge in repartee, street-talk, to have a rapport with.

(h) [1970s+] (US black) to talk with the aim of seduction.

3. [late 19C+] (orig. Aus.) to knock out, to kill.

In phrases

bad rap (v.)

[1960s+] (orig. US) to malign, to criticize unfairly.

rap to (v.) (US)

1. [late 19C] to own up to; to declare.

2. [1920s] to recognize; to work out; to discover.

rap up (v.)

see sense 2f above.