Boil1  —v. 1 a (of a liquid) start to bubble up and turn into vapour on reaching a certain temperature. B (of a vessel) contain boiling liquid (kettle is boiling). 2 a bring to boiling point. B cook in boiling liquid. C subject to boiling water, e.g. To clean. 3 a move or seethe like boiling water. B be very angry. —n. Act or process of boiling; boiling-point (on the boil; bring to the boil).  boil down 1 reduce in volume by boiling. 2 reduce to essentials. 3 (foll. By to) amount to. Boil over 1 spill over in boiling. 2 lose one's temper. [latin bullio to bubble]