1891, in reference to balloons, a coinage (with de-) based on inflate (q.v.). Latin deflare meant "to blow away," but in the English word the prefix has the sense of "down." Related: Deflated; deflating.
deflate a balloon
deflate the air mattress
The bad review of his work deflated his self-confidence
The new measures deflated the economy
deflate the currency
The balloons deflated