Green’s Dictionary of Slang

cock block v.

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

1. [1970s+] (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.

2. [1980s] (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’.

3. [1990s+] (US black/campus) ext. of sense 1 to imply interference in any plans, efforts.