a person who is plays in a band, typically a school band or ensemble. this term is sometimes used as an insult however it is also considered a term of affection as well.
Band nerds can be found in marching bands, pep bands, brass and woodwind ensembles, etc.
A put-down insult used by the untalented people who cannot play instruments intended to somehow "lessen the glory" of the talented, skilled people, but the put-down somehow fails to work every time, contrary to "orch-dorks", a put-down aimed at orchestra people which works every time.
I have to go practice the marimba, chimes, timpani, xylophone, triangle, tabourine, and gong!!!!!! TO THE BAND HALL!!!! dundundundunduuuuuun! (Band Nerd Alert!)