The "under" is the person at homosexualmethamphetamine parties that is so lethargic and strung out from shooting up, that he ends up being at the bottom of the orgy pile at all times, constantly getting railed.
The new strain of AIDS discovered back in September was traced back to a male who was the under at homosexual party.
Spending time in the Hennepin County jail in Minneapolis, MN. So named for the fact that the jail is located underneath the courthouse building which features a clock tower. The clock tower is also a well known Minneapolis landmark.