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?
by Lex Taliones January 17, 2010
A person who has nobody to love so they decide to take it out on you try and interfere with your relationship and stop the flow of love and emotions. They also prevent you from getting some....
Girl One: Baby, I love you

Boy One: I love you more
Cockblocker : *thinks to himself* I ain't get no pussy so I'll be a dickhead and stop these two....
*says aloud* who's hungry?
by Erectile Function October 29, 2014
someone who prevents someone from getting sex.
by Joey March 20, 2003
Cockblocker: 1. (noun) a female friend who accompanies another female friend to planned meetings with a male peer, to ensure that the sista does not get into trouble;
2. (Noun) a male who interferes with another male's attempt to freak the woman whom the former either wishes to protect as a friend, or is already pursuing himself;
3. (Verb) to cockblock: basically, to interfere, by way of one's mere presence, with the attempt of one person to freak another
1. Jill "Jane, I really need you to come along tonight as my cockblocker so I don't do anything I would later regret.
Jane "Sure"
2. I made my move toward that hottie Jennifer with the thought of grinding with her. That cockblocker Alex cut in and got it first.
3. Alex cockblocks all the time
by Doobie Smokes You December 2, 2004
A person who interferes with anothers chance of geting laid.
Spoon is the biggest cockblocker of them all.
by Anonymous October 5, 2003