doubtful adj.

c. 1200, doute, "uncertainty with regard to the truth of something," from Old French dote (11c.) "fear, dread; doubt," from doter (see doubt (v.)). The -b- was inserted later, as in the verb. Meaning "a matter of uncertainty" is from late 14c. Phrase no doubt "without question, certainly" is from c. 1400.

word-forming element attached to nouns (and in modern English to verb stems) and meaning "full of, having, characterized by," also "amount or volume contained" (handful, bellyful); from Old English -full, -ful, which is full (adj.) become a suffix by being coalesced with a preceding noun, but originally a separate word. Cognate with German -voll, Old Norse -fullr, Danish -fuld. Most English -ful adjectives at one time or another had both passive ("full of x") and active ("causing x; full of occasion for x") senses.

It is rare in Old English and Middle English, where full was much more commonly attached at the head of a word (for example Old English fulbrecan "to violate," fulslean "to kill outright," fulripod "mature;" Middle English had ful-comen "attain (a state), realize (a truth)," ful-lasting "durability," ful-thriven "complete, perfect," etc.).