"non-rigid airship," 1916, of obscure origin, with many claimants (even J.R.R. Tolkien had a guess at it). "One of the weird coinages of the airmen" [Weekley]. Common theory (which dates to 1919) is that it is from designers' prototype nickname Type B-limp, in the sense of "without internal framework," as opposed to Type A-rigid; thus see limp (adj.), but references are wanting. There apparently was a type b in the U.S. military's development program for airships in World War I.