Latin 'uniformis' (having one form) — the adjective preceded the noun for clothing by two centuries.
(Adjective) Not changing in form or character; remaining the same in all cases or at all times; consistent. (Noun) The distinctive clothing worn by members of the same organization or body.
From French uniforme or directly from Latin ūnifōrmis (having one form, of one shape, consistent), from ūnus (one) + fōrma (form, shape, appearance). The Latin ūnus derives from PIE *óynos (one, single), and fōrma is of uncertain PIE origin but possibly from *bʰer- (to carry, to bear — that which carries shape). The adjective sense (consistent, unvarying) came first and is recorded in English
Military uniforms were a surprisingly late development. Armies wore standardized clothing only from the mid-seventeenth century onward. Before that, soldiers wore whatever they had, sometimes with a colored sash or badge to identify their side. The noun 'uniform' did not appear in English until