Tenesmus

[Sarah]

An urgent and distressing sensation, as if a discharge from theintestines must take place, although none can be effected; -- alwaysreferred to the lower extremity of the rectum. Vesical tenesmus, asimilar sensation as to the evacuation of urine, referred to theregion of the bladder.

[Sarah]

An overpowering, incredibly painful sensation of your “backdoor” demanding to defecate (aka drop a deuce, colon cannonballs, butt driblets, digested Crayola box), with the inability to produce any colon candy (aka stool, feces, fecal matter, waste) despite a valiant, laborious effort.
Butthole Spasms, period cramps in your asshole
The anal equivalent of the urgency accompanied by a urinary tract infection. Gotta go, gotta go, gotta go right now… but you can’t.