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What one says/feels after they had amazing sex and/or a splendid fap session, the feeling is so intense you can barely stand up, thus weak in the knees.
Guy "Dude, you all right? your legs are wobbling"

Dude "Yeah I'm all right, I just received the best blow job from my gf; made me weak in knees "

"Damn, I jizzed so hard last night my legs couldn't support me for the next 3 minutes."

"Damn, I banged my gf so hard last night I thought my knee caps would explode"

weak in the knees
by SirCumsALot April 13, 2013
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