Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
burn out (v.), burnout, burn-out (n.), burned-out, burnt-out (adj.)
To burn out is an idiom meaning “to stop burning because of lack of fuel”: The fire had burned out sometime during the night. Burnout is the usual spelling of the noun, burn-out an infrequent variant: it is the point at which a booster rocket has consumed all its fuel and is dropped away from the next stage, or the point at which a jet engine stops after burning all its fuel. Burnout is also a figurative term for emotional exhaustion stemming from overwork or other on-the-job strain, as in A newly recognized educational problem is teacher burnout, and burned-out or burnt-out is the adjective of this sense.
