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A person that acts without any care of what'll be the consequences of what he's doing. Also a stupid, superficial, dull person.
1. Oh, damn! You act like you're brainless! Don't you think about what will happen if you do that?

2. That boy's brainless. I suggest you to not go out with him.
by KD Squib August 28, 2005
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2
People with no brain, idiot, dumb...
Or, people dressed like homeless listening bad reggea, anytime sleeping, with antique shoes (nastase for exemple), and who have a strap in his hair, like youg school girl...
and who is a looser at 'trivial pursuit'.
toumeuh is a 'brainless'.
by pozi October 19, 2006
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3
A backflip with a 180 degree turn on a ramp, a rollerblading trick.
That grommet just did a brainless!
by Kody_Bank's_Girl November 24, 2005
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4
An old english term for academics who herald from Marmesbury. (A town in England) a "jonny runny poo" is a rare breed. Usually found in any university library, flicking through back issues of country life, this creature adores all things Wildean, debauched and longs to marry a ho ming from Pobo.
Come here Jonny Runny Poo, stop being so brainless.

by FOSHIZZLEMANIZZLE October 30, 2006
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5
A terrible thing that happens when born out of a womb that is not ripe. This is a very rare disease that happens 1 in 1,000,000,000,000,000,000,000,000 people Only un-dope parents can have this problem. It depends how much time they spend on the internet and watching TV.
"HOLY S%^& ITS BRAINLESS, A BRAINLESS CHILD!? YOU MUST BE THE MOST UN-DOPE PEOPLE IN THE UNIVERSE"
via giphy
by WordsAreWeird101 May 29, 2017
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6
someone who lacks in intellect or chooses not to use their intellect and acts carelessly without making any mental connections, developing perspective or using foresight .
a gardener , while trimming her shrubberies became brainless when she saw a group of hot guys approaching her and ended up cutting a big hole in the middle .
by tiredbrokenrecord June 08, 2016
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