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One is a member of Thumb Club having successfully Fingered but in place of a finger, using your thumb. Often designated by giving the "thumbs up" sign.
"Hey mang, I was hanging with my girl last night and I got into the Thumb Club!""
by Boognish Rising August 06, 2008
The club one joins after having inserted his thumb into his wife or girlfriend's anus while have doggy style intercourse.
Dude, I joined the thumb club with this chick last night when I was pounding her doggy style.

I joined the thumb club with this girls last night. Check out my chocolate wedding ring. lt came with a corn kernel diamond.
by Dr. Brown finger November 09, 2013