Forgetful

wiktionary

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From Middle English *forgetful, *forȝetful (suggested by derivative forgetfulnesse, forȝetfulnesse(“forgetfulness”)), equivalent to forget +‎ -ful.

etymonline

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forgetful (adj.)

late 14c., from forget + -ful. A curious formation. Used in the sense "causing forgetting" from 1550s, but almost exclusively in poetry (Milton, Tennyson, etc.). An older word in this sense was Middle English forgetel, from Old English forgitel "forgetful," from a formation similar to that in Dutch vergetel. Related: Forgetfully; forgetfulness.