"Spats" is short for "spatterdashes" which are "half-gaiters" - abbreviated versions of the full-length gaiters worn by military men, and some civilians (especially sportsmen), to protect their hose from getting mangled and soiled while in the field, as well as to keep debris out of the shoes.
As the 18th C. wore on, the full-length gaiters tended to be replaced by the shorter "spatterdashes" for both military and civilians. Kilted Highland troops never wore the full-length gaiters as an issued part of their kit (though many "acquired" them for use in N. American forests!), but adopted the shoter "spats" in the latter 1700s....
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