The term 'Fire on all cylinders' is a metaphor that means to give something your full effort. It originates from the internal combustion engine, where cylinders are the parts of the engine that generate power. The term HUM, which is a 3 letter word, is an acronym for 'Hear, Understand, and Memorize'. This phrase is used to remind people to listen carefully, understand what they are hearing, and remember it.