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It basically only means understand. Its a way to make yourself sound like a 6 year old.
"Dude: Hey, sorry I couldn't go for a walk with you today. Mom wanted me to go to town."

"Dudette: Yeah its all right, I underconstumble."

"Dude: what are you, six?"
by BranMuffin11 September 05, 2008
To understand, mostly. Something you say when you think you've almost got it, almost understand.
After being told to remember a long list of directions to get to the airport, she said, "I underconstumble".
by Larke September 06, 2008