Jell

google

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mid 18th century: back-formation from jelly1. See also gel1.

wiktionary

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A back-formation from jelly; reinforced by gel. Compare obsolete geal.

etymonline

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jell (v.)

"assume the consistence of jelly," 1869, American English, probably a back-formation of jelly (n.). Related: Jelled; jelling. Figurative sense is first attested 1908. Middle English had gelen "congeal," but it disappeared after 15c.