“Your Mother” refers to possibly the dumbest and most overused, not to mention least original, in-joke discussed in the UGP. Originally a Andrew-Rachel phenomenon, it then spread to Nick and Kevin, and is grudgingly tolerated by Nick’s other friends. Chris grudges especially on this one.
The joke consists simply of taking some idle turn of phrase that might possibly have sexual implications, associations, etc., and then associating those implications or associations with the mother of ones interlocutor. It is a popular joke elsewhere, and is only an in-joke inasmuch as we’ve gone much farther with it than most people have gone with anything except your mother.
That right there is indicative both of the moronic character and of the low quality of our “Your Mother” jokes.
The joke has died off somewhat of late, having lost much of whatever amusement it ever afforded us. As new or historical occurrences occur, they may be added to the “quotes” section below.
“Your Mother” jokes as used here should not be confused with the popular “Yo Momma” breed of joke, which most typically runs to the form “Yo Momma so fat…” Ours are to be distinguished by their exclusively sexual content, and by their operation purely on the basis of reference to (ostensibly) non-intended turns of phrase.

[Quotes]:
  • During a discussion of Eternal Darkness, Andrew: You just said “Internal Darkness”...I’ve seen your mother’s internal darkness!
  • wackyslav: shrugs Sounds like our kind of guy, only more computer-oriented.

    Elmo Crmley: Maybe.

    Elmo Crmley: He seems a little more anal.

    Elmo Crmley: So, more your mother’s kind of guy.