undoubtful

Undoubtful is an adjective that is rarely used in modern English. It is traditionally understood to mean not in doubt, i.e., certain or free from doubt. In contemporary usage, the form is generally regarded as archaic or nonstandard; most writers prefer undoubted or unquestionable to express certainty, or use undoubtedly as the adverb.

Etymology and form: the word is formed with the prefix un- attached to doubt plus the suffix -ful. The root doubt comes from Old French doute and from Latin dubitare, with related terms across English and other languages. The historically constructed form mirrors other negations of adjectives such as hopeful or careful, but its modern status is largely literary rather than common.

Usage and style: historical texts sometimes employ undoubtful to modify a noun, for example in older manuscripts. In current dictionaries, undoubtful is usually absent or marked as obsolete or nonstandard. When aiming for clear, standard English, speakers and writers typically replace it with undoubted or unquestionable, or rephrase to avoid the adjective altogether. If one intends to convey a lack of doubt in the sense of confidence rather than mere certainty, alternatives such as undoubting or confident are more natural. The common adverbial form is undoubtedly.

Relation to other terms: undoubted and unquestionable are common synonyms for "certain." Doubtful remains the opposite. The standard adverb form is undoubtedly, not undoubtfully.

See also: doubt, doubtful, undoubted, unquestionable, certain, undoubtedly.