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.)