When resen builds up on the downstem of a bubbler or a bong resembling barnacles.
Aw nasty dude! How much pot did you smoke to get those bongacles?!
A bongacle is the resen build up on any kind of downstem on bongs and bubblers with downstems. They start collecting like little barnacles and look pretty rancid because they have been in your bongwater for a while.
Aw dude! You need to clean out your bong, those bongacles are huge!