Wordorigins.org

This term for a non-rigid airship is of uncertain origin. We do know that it was coined during the First World War, but who coined it and why the rather enigmatic term blimp was chosen may never be known. Making matters worse, the various origin stories are often conflated in various sources, making sorting out the truth difficult.

First, the earliest known use of the term dates to February 1916, from Rosher’s In R.N.A.S.:

Visited the Blimps...this afternoon at Capel.