singlet n.

early 14c., "unmarried," from Old French sengle, sangle "alone, unaccompanied; simple, unadorned," from Latin singulus "one, one to each, individual, separate" (usually in plural singuli "one by one"), from PIE *semgolo‑, suffixed (diminutive?) form of root *sem- (1) "one; as one, together with."

The meaning "consisting of one unit, individual, unaccompanied by others" is from late 14c., often merely emphatic. The meaning "undivided" is from 1580s. Single-parent (adj.) is attested from 1966.

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]

Proto-Indo-European root meaning "one; as one, together with."

It forms all or part of: anomalous; anomaly; assemble; assimilate; ensemble; facsimile; fulsome; hamadryad; haplo-; haploid; hendeca-; hendiadys; henotheism; hetero-; heterodox; heterosexual; homeo-; homeopathy; homeostasis; homily; homo- (1) "same, the same, equal, like;" homogenous; homoiousian; homologous; homonym; homophone; homosexual; hyphen; resemble; same; samizdat; samovar; samsara; sangha; Sanskrit; seem; seemly; semper-; sempiternal; similar; simple; simplex; simplicity; simulacrum; simulate; simulation; simultaneous; single; singlet; singular; some; -some (1); -some (2); verisimilitude.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sam "together," samah "even, level, similar, identical;" Avestan hama "similar, the same;" Greek hama "together with, at the same time," homos "one and the same," homios "like, resembling," homalos "even;" Latin similis "like;" Old Irish samail "likeness;" Old Church Slavonic samu "himself."