etymologeek.com dictionary
English word thankful comes from English thank, English -ful.

Thankful etymology history

Thankful detailed word origin explanation

Dictionary entryLanguageDefinition
thank English (eng) (obsolete) An expression of appreciation; a thought. (transitive) To credit or hold responsible.. (transitive) To express gratitude or appreciation toward.. (transitive) To feel gratitude or appreciation toward.
-ful English (eng) Used to form adjectives from nouns. Full of, tending to, or thoroughly possessing the quality expressed by the noun.. Used to form nouns from nouns meaning “as much as can be held by what is denoted by the noun”. Used to form nouns indicating a great deal of the quantity expressed by the noun.
thankful English (eng) (obsolete) Obtaining or deserving thanks; thankworthy.. Showing appreciation or gratitude.

Words with the same origin as thankful

Descendants of -ful
colorful delightful disgraceful doubtful graceful grateful hateful helpful hopeful hurtful joyful meaningful merciful painful peaceful resourceful respectful rightful stressful successful suspenseful truthful useful youthful