No Apparent Reason Boner. When you're just sitting there taking a test or listening to a sermon in church and for some random reason, you get a raging hard-on. Hopefully you are wearing tight jeans or tighty-whities and not gym shorts.
I finished with my test early but had to wait to leave because I didn't want to jab anyone in the back of the head with my Narb as I was leaving my seat.
Acronym for "No Apparent Reason Boner", often the bane of male junior high students and some high school students everywhere. They seem to occur at the most inopportune moments such as right before the bell rings and when a teacher calls you to the board or their desk.
Wes: "Hey everyone look, JT has a NARB!!!" (Everyone points and laughs.)
JT: "Faggot..." (Covers boner with chemistry book.)