The story
- Old English · inherited fromþancful
- Middle English · inherited fromthankful
- English todaythankful
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”).
Etymology from Wiktionary, CC BY-SA 4.0.