Extremely large brain. Usually named Lily.

Basically a person with a very large brain that has hella smart stuff.
Random dude: what’s 42+27?
Lily: 69 🧐
Random dude: damn you hella big brain
by Edjn October 24, 2019
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Big-brain, (Which can also be spelled without a hyphen,) means smart, intelligent, and is usually used when someone says something witty in an argument.
Maddie is really big-brain for saying that!
Maddie is really bigbrain for saying that!
via giphy
by ~Chrysanthemum~ December 02, 2020
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