google
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early 18th century (in the sense ‘boggy’): from dialect sog ‘a swamp’ + -y1.
wiktionary
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From sog + -y.
etymonline
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soggy (adj.)
1722, perhaps from dialectal sog "bog, swamp," or the verb sog "become soaked," both of unknown origin, + -y (2). Related: Soggily; sogginess.
