mid-14c., "type of tight-fitting men's outer garment covering the body from the neck to the hips or thighs," from Old French doublet (12c.), from diminutive of duble "double, two-fold," from Latin duplus "twofold, twice as much" (see double (adj.)).
From 16c. to 18c. doublet and hose meant "typical male attire." From 1550s as "one of two things that are alike," originally of words in the same language differing in form but from the same ancestral word.
In philol., a duplicate form of a word ; one of two (or, by extension, three or more) words originally the same, but having come to differ in form, and usually more or less in meaning. Doublets are very common in English. They usually consist of an older and a later form, the older being generally descended and the later directly borrowed from the same original (as benison, benediction; malison, malediction, etc.), or two accidental variations of one original, sometimes slightly discriminated (as alarm, alarum, etc.), or of a standard literary and a dialectal form (as church, kirk; lord, laird, etc.). [Century Dictionary]