Inflame with enthusiasm, anger, or another strong emotion, as in Her speech fired up the crowd in favor of her proposals. This expression dates from the early 1800s, when it referred literally to starting a fire in a furnace or boiler; its figurative use dates from the late 1800s.
Light a pipe, cigar, or cigarette, as in Do you mind if I fire up? [Late 1800s] A more common term, however, is light up, def. 2.
Start the ignition of an engine, as in Whenever he tried to fire up the motor, it stalled. [Mid-1900s]