(1) A blowout is a spontaneous cleaning of the digestive system caused by eating
the wrong foods (if by accident) or the
right foods (if done on purpose). Its effects on hiking companions can range from severely annoying to fatal. Really bad blowouts can destroy small tents (or apartments, when they happen indoors). A particularly violent type is known as
Blowout And Cratering (B&C).
(2) A blowout can also be a shallow depression in a sandy or dusty place caused by wind action removing soil and possibly some plants. Or by cleaning out of the digestive system.
(3) Another form of blowout is a party at the end of the trail, inspired by the realization that no one has to do any more of that walking stuff with the bugs and dust and all. Such a party is frequently accompanied by muscle relaxant (CH3CH2OH) and loud, rude, irreverent noises. And sometimes by a cleaning out of the digestive system due to the ingestion of large quantities of unfamiliar (i.e., real) food.
Source: How to talk in the woods.