Thankful

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Old English thancful (see thank, -ful).

wiktionary

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From Middle English thankful, from Old English þancful, þancfull(“thoughtful, pleasing, agreeable, pleasant, thankful, grateful”), equivalent to thank +‎ -ful. Compare Old High German undankfol(“unthankful, ungrateful”).

etymonline

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thankful (adj.)

Old English þancful "satisfied, grateful," also "thoughtful, ingenious, clever;" see thank + -ful. Related: Thankfully; thankfulness. Thankfully in the sense "thankful to say" is attested by 1966, but deplored by purists (compare hopefully).