An erection caused by the stroking of real or fake fur. Very common in people with fur fettishes.
Earl: Renise wears her ugly llama jacket all the fucking time. It's so disgusting.
Annie: Oh I know, and it's especially disgusting when she strokes it and gets an erection.
Edna: It's called a furection, silly.
An erection lasting a few micro-seconds. This is invisible to anyone but the trained eye. It is an involuntary muscle reflex.
Not to be confused with a "stirring in the loins" - a more labored and deliberate feeling, involving a certain degree of participation from the subject.