The story
Clipped compound: clipping of military specification. Originally capitalized (MIL-SPEC) in the extensive lexicon of initialisms, acronyms, and clippings of the U.S. (and other) militaries and defense industries, which began in the nineteenth century but greatly proliferated in the mid-twentieth century and has remained large and growing since then. More at Wikipedia under United States Military Standard.
Etymology from Wiktionary, CC BY-SA 4.0.