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1.
a. When a man wears really small loose shorts and part of his ball bag falls out he is considered to have testicleavage.
b. When an exposed ballsack is observed by the general public, usually by older women who will then scream and run away.
Jane saw Johns testicleavage when he bent over to tie his shoe while wearing a speedo.

Often times testicleavage is observed in the locker room of the swim team
by Nalina & Adam April 27, 2007
 
2.
The slight area of ascension between the testicles, as they hang loose. This is considered the "cleavage" of a pair of testicles.
Bella saw Povaliev's testicleavage when he got naked for her.
by TheRichandBeautifulTJF! March 30, 2010