Sphincter - Anatomically a circular muscle that can constrict or relax in order to open or close a passage.
Sphincter - In squaddie terms almost always used in conjunction with the anus. The anal sphincter is the final barrier to kicking a chicks Back Doors in and indulging in some bum sex in order to win his Brown Wings.
The part of the arse that is glued together after the consumption of Biscuits AB (Biscuits Arse Blocking) on exercise. After consuming Biscuits AB the sphincter can only be released by unleashing a Compo Anaconda on the unsuspecting muscle.
When sober a vital part of the arse mechanism that contains the contents in of the arse in the correct storage locker prior to the Unload. When Trollied after the consumption of 15 pints of wifebeater or wobbly followed by a kebab, this mechanism can unexpectedly fail with disastrous results.