The dreadful (yet metaphorical) place where a lady that takes a man's fancy resides when he forgets to kiss her goodbye after a date, or generally behaves in a way that will prevent said female from ever jumping into bed with him. Tell tale signs include introductions as 'some guy,' and gagging gestures whenever he tries to flirt.
To the average male, it is slightly worse the seventh circle of hell.
Dave: I think i've found myself a lovely, sexy, hot, female 'friend.' FUCK!
Andy: Why don't you just rag the shit out of her?
Dave: We're in the friend trench.