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75.
When you're about to score but someone ruins it for you.
Can be ruined for one or both parties.
Mood killer.
Best described with an example.
Haley and her boyfriend Andrew were having phone sex when Andrew's brother James walked in.

Andrew: Then I'll take off your shirt and your bra and bite your neck and pull your hair
Haley: I'll take off your shirt and your pants and get on top of you and start biting your neck softly
Andrew: Then I'll move down to your stomach and take your panties off and I'll start to eat out your pussy

Haley: Next I'll take your boxers off and kiss your dick, I'll get on my knees and start sucking your cock
Andrew: Mmmmmm, Baby, what's next?
Haley: I think that's up to you;)
Andrew: I want you to suck me until I'm rock hard and about to explode. Then I'll get on top of you and pin you to the bed and start to put rock hard cock in your tight, wet....
James: Hey man, what's up?
Andrew: You fucking cock block!!!
James: Who are you talking to?
Andrew: Haley!!! We were having a good time!!
Haley: Tell you're brother he fucking ruined it!!!
James: YOU'RE WELCOME
Andrew: Get out James!!! Baby, I'm not horny anymore. I'm sorry my brother's a fucking cock block.
Haley: Ugh, neither am I. Tell him that I'm fucking pissed at him.
by LoveLoveLove27 July 06, 2010
 
1.
One who prevents another from scoring sexually.
Trudy and Dan frequently had time alone but her cockblocking roommate ruined any chances they had to have sex.
by Miss Priss April 13, 2003
 
2.
1. To prevent a male friend or associate from getting some action.

2. Any such attempt to prevent a male friend or associate from getting some action.
1. Damn, Johnny just totally cockblocked his roommate. Ass.

2. OK, Isaac's making his move on Clarisse in the living room, and the game is on in fifteen minutes. We're going to have to attempt a cockblock.
by Jake April 12, 2003
 
3.
1.) To interfere with someone who is getting acquainted with, conversing with, or hooking up with a member of the opposite sex

2.) A motor vehicle maneuver, usually done by insecure twenty-something males (who think that everybody wants to race them) and other ignorant people, designed to prevent someone from changing lanes in traffic
1.) At the club, Sal was was about to get Lisa's phone number, when Nathan came up cock-blocked him by asking him if he found a job yet. He made it even worse when he reminded Sal that they had to hurry up and catch the last bus home. Lisa rolled her eyes and walked away.

2.) I realized that my exit was coming up so I began to change lanes, but that ricer cock-blocked me

3.) The traffic on the highway was hell, mostly because of that soccer mom driving the minivan in the far left lane, pacing and matching speeds with the semi in the middle lane. She cock-blocked us all.
by TheGrandInquisitor January 24, 2005
 
4.
1.The act of obstructing one person's advances towards another.
2.A situation that obstructs; an obstacle
3.An act of ill etiquette in which a male is speaking to one female in a group of females, and the alpha female creates a disruptive environment.
She is cockblocking you bro.
by The GIMP September 23, 2003
 
5.
a person who interferes with one's sexual intentions with another person
Randy was a cockblock because he told Jaime that this guy she knew was just using her for sex.
by Bob Mastuff October 13, 2003
 
6.
(n.) one who prevents another from getting "play", "poontang", etc.
(v.) any action that impedes or stalls another's "game", "mack", or "pimp manuver"
(n.) I was dancing with this girl, when this cock-block stepped in between us and started grinding with her.
(v.) I had finally convinced Susan to go to my friend's keg party with me, when my Dad cock-blocked me by taking away my car keys.
by MCKREWGA September 13, 2003
 
7.
1. To interfere with a man's attempts to seduce a woman.

2. abbr. Cockblocker(n.)
I was putting the move on this chick and Ralph showed up and cockblocked me.
by Anonymous February 06, 2003