1776 as a musical term, from Italian crescendo "increasing," from Latin crescendo, ablative of gerund of crescere "to increase" (see crescent). Figurative use is from 1785. As a verb, from 1900.
A musical direction used to indicate increasing loudness.
Note: The term is sometimes used figuratively to indicate rising intensity in general: “As the days went on, there was a crescendo of angry letters about my speech.” Crescendo is also sometimes misused to indicate a peak of intensity, as in, “The angry letters about my speech hit a crescendo on Wednesday.”