dictionary.com:
Daring or
reckless action
Not to be confused with herring-do, which involves a small fish, a gallon of oil, and very sore buttocks afterwards.
This is the type of word/phrase scattered throughout those horrible books Freshman English teachers in American high schools delight in forcing upon their charges, muffling cackles with a mildewed copy of Giants in the Grass or A Separate Peace.