1. A man... or woman with only one testicle which makes it incredibly hard to ride a whorse
2. A person who lacks true manlyness and attempts to mask his insecurity by carrying out idiodic tasks. (in this sense it is similar to the cliche of putting all your eggs in one basket)
Uni-testicle is referred to a person who's bad at gaming. This is a word invented by "Zemog" - a player on Xbox live player notorious for talking nonsense online.
Uni-testicle quickly spread around various sites and became a slang for someone worse then the person using the word.
I have a uni-testicle (Pronounced; Uuuuunitesticle.)