When the penis becomes flaccid. This is much like the term nobon, but obviously with the suffix "on" removed and a new suffix of "off" replacing it.
For some males a "noboff" can be a repeating problem. An example of a "noboff" becoming a problem would be trying to engage in sexual intercourse. This is normally when some men choose to turn to viagra.
Lenny: "Where's the viagra bitch. I've got a noboff"
Lisa: "Leave me alone you pervert."