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1.
Being on the same wavelength as someone, being mentally in-synch with someone, or working really well together.
My GF and I were just not gelling today. She wasn't getting any of my jokes.
by mangodaiquiri October 01, 2009
 
2.
Another word for masturbating
Guy 1: Hey dude, what were you just doing in there?

Guy 2: Ah, nothing. Just gelling.

Guy 1: I see. I might go do some gelling later too.
by A mothafuckkin starfish December 20, 2013
 
3.
If you are *not* wearing Doctor Sholls extra soft insoles then you tell others you are "gellin'"
"Im gellin"
"dude, your so not gellin'
by backstab.net September 07, 2003