Green’s Dictionary of Slang

cock block v.

also c.b.
[cock n.3 (1)]

1. (US black, also block) to ruin another man’s sexual activities by stealing his woman, interrupting his seduction, etc; thus cock-block, cock-blocker, one who does this.

[US]E. Folb Urban Black Argot 135: Cock Block to interfere with a male’s attempt to ‘win over’ a female, even if the other male is not interested in the female himself.
[US]E. Folb Runnin’ Down Some Lines 139: You both jus’ gonna sit there and cock block and neither one o’ you gonna get nothin’. [Ibid.] 231: c.b. Abbreviation for cock block.
[US]Eble Campus Sl. Fall 2: cockblock – [...] I was talking to Jenny, and James cockblocked me by calling her to him.
[US]Ultramagnetic MCs ‘Porno Star’ [lyrics] Tell your friends to leave, no time for cockblockers.
[US]G. Smitherman Black Talk 61: block To interfere with a man who’s hittin’ on — i.e., trying to establish a relationship with — a woman. Euphemism for cock block.
[US]Nasir Jones ‘Virgo’ [lyrics] But watch for cock blockin all my coochie stock options.
http://www.returnofkings.com 27 Feb. [Internet] Cockblockers still break down into the same traditional taxonomies. Some are in-house (your own ‘friends’), others are external (her friends), and still others are environmental (strangers).
C. Eble (ed.) UNC-CH Campus Sl. Fall 3: COCK BLOCK — thwart an amorous opportunity for another : ‘Dude, you cock blocked me at that party’ ; ‘I really wanted to talk to her, but her friend was cock blocking’ Someone who interferes with another’s attempt to connect for sex or romance: ‘I was trying to get with her, and you were such a cock block!’.

2. (US) in a sadomasochistic scenario, ‘to bind the penis, as with nylon stocking, in such a way that binding can be tightened quickly to prevent ejaculation’.

[US]personal ad, restroom Murray & Murrell Lang. Sadomasochism (1989) 53: Experienced dominatrix specializes in training male slaves with cockblocking.

3. (US black/campus) ext. of sense 1 to imply interference in any plans, efforts.

[US]Eble Campus Sl. Fall 2: cockblock – come in from an unseen angle to prevent someone from doing something.
[US]Ebonics Primer at www.dolemite.com [Internet] cock-block Definition: negatively interfering in a situation where one is trying to achieve greatness. Example: Ah man, I was standin’ on the corner slangin’, and his ol’ cracker got the police on me. Shit, those peckerwoods always got to cock-block niggas.
Buckshot & 9th Wonder ‘Ghetto’ [lyrics] Every day it’s a cop or cock-blocker.
[US]D. Winslow The Force [ebook] Sykes doesn’t bring McGivern in on this from the jump, the inspector will cockblock his career until he pulls the pin.