adverb. Extremely so--in a crushing/vacating the bowels way.
Real citation: "Previous New Years come to mind in a slurry of bowel-crushingly embarrassing memories. Blowing chunks in a bin outside Leatherhead Leisure Centre, falling asleep at 10.30pm in an airing cupboard ripped to the gills on Malibu, weeping into a vol-au-vent at the passing of another year. To be honest, most of my New Years have been spent with my parents, rugs on knees and sherries in hand, watching people in Scotland on the telly having a really brilliant time."
("Previous New Years come to mind as bowel-crushingly embarrassing memories," http://www.melandsue.co.uk/archive/articles/20_11_99.htm)
Made-up citation: "What bowel-crushingly moronic decisions will I make in the year ahead? Me and my bowels (and my underwear) can't wait to find out."