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.