A violent bowel movement, usually loose, which the person
may have been storing for some
time due to lack of access to ammenities. Propelled by a buildup of
gas caused by the fact that a fart during this
time would have resulted in a follow through.
It is characterised by shitting out your entire lower colon (and, possibly, a kidney or two) in the space of 2 to 3 seconds.
The result leaves the thunderdumper feeling up to half a stone lighter, noticeably weak, but relieved.