A coffee crush is interpreted as the inexplicable urge to visit a female (or male) who works at the local coffee shop repeatedly even when not in a current state of craving coffee. Usually there is no real motive behind the visit other than to see the employee in which you have the crush on.
Side effects may include: Finding out through conversation that your crush has a significant other, having your drink ready before you reach the window hence less time for conversation, lonely nights, etc...
Male 1: Geez, you filled up your stamp card in 3 days!
Male 2: Yeah... I kind of have a coffee crush...