A cockblock is a slang term for an intentional, or sometimes unintentional action that serves to prevent someone from having sexual intercourse with a partner.
Such behavior is often motivated by jealousy, or competitiveness, although it is sometimes accidental, or inadvertent. A cockblock, or cockblocker, is a person who engages in such obstruction or intervention.
Social research has documented norms among male peer groups that view "cockblock" behavior as negative, which may make men less likely to intervene as bystanders in situations of interpersonal violence. The terms appears to date at least to 1972, when Edith Folb documented its use by urban black teenagers.
^Folb, Edith. Runnin’ down some lines, Society, Volume 18, Number 1 (1980), 63–71 ("There are a number of expressions in the vernacular that characterize one person's attempt to interrupt, impede, or totally sabotage another's action – to cock block ...")