Solitary

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Middle English: from Latin solitarius, from solus ‘alone’.

wiktionary

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From Middle English[Term?], borrowed from Latin sōlitārius. Doublet of solitaire.

solitary

etymonline

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solitary (adj.)

mid-14c., "alone, living alone," from Old French solitaire, from Latin solitarius "alone, lonely, isolated," from solitas "loneliness, solitude," from solus "alone" (see sole (adj.)). Meaning "single, sole, only" is from 1742. Related: Solitarily; solitariness. As a noun from late 14c.; from 1854 as short for solitary confinement (that phrase recorded from 1690s).