have a bee in one’s bonnet To be obsessed by a delusive notion or fantasy; to be preoccupied by a whimsical or perverse fancy; to be eccentric or crotchety. Variants of this phrase, such as bees in the head or brains, and maggots in the head or brains, were used as early as the beginning of the 16th century, although bee in one’s bonnet is heard almost exclusively today. As for its origin, it seems evident that anyone with a live bee buzzing inside his hat would be preoccupied indeed. Perhaps the use of alliteration accounts for its currency.

one-track mind A mind completely obsessed with a single thought, idea, or desire; an extremely narrow point of view. This common expression alludes to a single set of railroad tracks on which trains can move in only one direction. As used today, the phrase often carries the disparaging implication that the possessor of such a mind stubbornly resists any consideration of alternative viewpoints.

ride a hobbyhorse To pursue a favorite project or idea relentlessly and unceasingly; to be obsessed with a single notion or scheme. A hobbyhorse was the term given to a wickerwork horselike frame used in the old Morris dances, as well as to the stick toy with a horse’s head ridden in mock fashion by children. The expression originally meant to play an infantile game of which one soon tired, since riding such a hobbyhorse involved little more than monotonous repetition of unvaried movements. In the 1700s John Wesley referred to hobbyhorse as “the cant term of the day.”