A male who unwittingly(out of ignorance) or purposefully(out of jealousy) prevents another male from engaging in debauchery with members of the female sex. Females can also cockblock, but the true essence of the word lies in the metaphor of the penis used figuratively as an impediment which thwarts any possible chances of scoring with members of the opposite sex.
friend # 1 interrupting friend # 2, who is conversing with a female at a local bar.
"Hey i don't mean to interrupt, but did you ever get the results of those STD tests that you took?
"Hey, does your Wife know where you are tonight?"
" Does your probation officer know you are here tonight?"
'What we talked about before, your experiences with Thai She Males... it's totally between us. carry on!"
"Friend # 2-- Boy, you really are a cockblocker, aren't you?