late 14c., "causing doubt, not distinct in character, meaning, or appearance," from doubt (n.) + -ful. From c. 1400 as "of uncertain issue, precarious." From early 15c. as "full of doubt, having doubt, hesitant, wavering." By mid-15c. as "admitting or subject to doubt." Related: Doubtfully; doubtfulness.
Other words that have been used in English in some or all of these senses were doubtous "undetermined" (mid-14c.); doutive "filled with doubt" (late 14c.); douty "ambiguous, enigmatic, obscure" (late 14c.); doubtable (c. 1400); doubtsome (1510s).