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Phrase(s): a pain in the ass [and] a pain in the butt; a pain in the rear
Fig. a very annoying thing or person. (Crude. Potentially offensive. Use only with discretion. An elaboration of pain. Use caution with ass. Butt is less offensive. Rear is euphemistic.) • That guy is a real pain in the ass. • Things like that give me a pain in the butt.
McGrawhill's American Idioms And Phrasal Verbs