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simply it means when you're in the stage of life that comedian Danny Thomas referred to as "young and foolish". In the hit "New World Man" the band Rush details this further with the verses:

He's old enough to know what's right but young enough not to choose it

He's wise enough to win the world but fool enough to lose it

He's a New World Man...
I was at the souvenir tent drinking a Coke before Sevara hit the stage to open for Peter Gabriel that night when I ran into Bryan. He was showing the girls his tattooed legs. His tattoos were symbols of all the rock concerts he'd been to so far.

There was the tongue + lip for the Rolling Stones, a prism for Pink Floyd and so forth. One blonde beauty pointed to a hand of playing cards that said "U2" on them, asking how is U2 associated with poker. Bryan simply deadpanned, "I was young, dumb and full of cum".
by I Saw U2 Live Twice February 20, 2011
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young, dumb and full of cum is referring to girls who are find their snatch is cum filled to the brim after a night on the town being shared around.

These particular sluts don't use protection and are too dumb to know better or care.
These girls often view their STD's as trophies confirming their sexual promiscuity status as rock star, collecting them like a girl guide collects badges for her sash and status.
After falling pregnant as a result of spontaneously hosting a (Bukkake) Jenny finds herself unsure who the father out of the 15 guys whose (loads) she took like cannonballs to her cunt.

In (hindsight) she agrees that she was young, dumb and full of cum that night.
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by rapunzabel September 15, 2016
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3
young, dumb and full of cum is referring to girls who are find their snatch is cum filled to the brim after a night on the town being shared around.

These particular sluts don't use protection and are too dumb to know better or care.
These girls often view their STD's as trophies confirming their sexual promiscuity status as rock star, collecting them like a girl guide collects badges for her sash and status.
After falling pregnant as a result of spontaneously hosting a (Bukkake) Jenny finds herself unsure who the father out of the 15 guys whose (loads) she took like cannonballs to her cunt.

In (hindsight) she agrees that she was young, dumb and full of cum that night.
via giphy
by rapunzabel September 15, 2016
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4
When one gets home from school and thinks about doing homework for math class, then goes to look up what does Calculus really mean on the internet, but then he remembers that extremely hot chick in class with the cheerleader outfit, that was giving him the I wanna fuck stare, while pulling up her skirt a little to reveal her perfect gams, and then he gets severely distracted by the gigundous amount of porn there is on the nets, and decides instead to spend the rest of the evening shooting a load off. Then after shooting off waterfalls of cum, passes out and completely forgets he even had any homework at all.

In other words it means fulfilling your inner animal need to release sperm, or thinking with your dick, instead of using your intellect to figure out things like math, in the thinking fucking part of the brain.
Math Teacher: "Ok everyone, turn in your homework, oh guess what we have a surprise test today!"
McFly: "Aw shit I spent the night looking at porn and beating off, cause Mary Jane Rottencrotch keeps giving me the fuck me eyes in her cheerleading uniform."
Biff: "Oh man well you're fucked. I guess you forgot to do MY homework too, man quit jerking off McFly."
Mary Jane Rottencrotch:"Were you thinking of me when you were spanking?"
McFly:"Dammit quit being so hot. Why can't I just do my homework instead of slapping the salami all the time?"
Math Teacher: "That's because you are young, dumb and full of cum."
by CrotchFinger September 05, 2016
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5
young, dumb and full of cum is referring to girls who are find their snatch is cum filled to the brim after a night on the town being shared around.

These particular sluts don't use protection and are too dumb to know better or care.
These girls often view their STD's as trophies confirming their sexual promiscuity status as rock star, collecting them like a girl guide collects badges for her sash and status.
After falling pregnant as a result of spontaneously hosting a (Bukkake) Jenny finds herself unsure who the father out of the 15 guys whose (loads) she took like cannonballs to her cunt.

In (hindsight) she agrees that she was young, dumb and full of cum that night.
via giphy
by rapunzabel September 15, 2016
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