Singleton

google

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late 19th century: from single, on the pattern of simpleton .

wiktionary

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From single +‎ -ton, probably following the model of simpleton.

etymonline

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singleton (n.)

"single card of a suit in a hand," 1876, originally in whist, from single (adj.); compare simpleton, etc. Extended early 20c. to other instances of singularity.