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1.
A derpi is the coming together of a Jessi and a gussi usually in an online game form.
A derpi may also be used as an affectionate term of endearment by one of the aforementioned or by any other user. The term derpi stems from the origin, derp.
One might refer to another individual as a "derpi" if they have done something silly.
by DerpiJessi August 22, 2012
 
2.
The bottom half of the reproductive organ on the women, or the vagina.
"I fucked her derpis so hard last night"
by CaptainFaggot February 03, 2012