Etymology
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doubtful (adj.)

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).

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dubious (adj.)

1540s, "puzzling, occasioning doubt or uncertainty;" 1630s, "doubtful, hesitating in opinion;" from Late Latin dubiosus "doubtful," from Latin dubium "doubt," neuter of dubius "vacillating, moving two ways, fluctuating;" figuratively "wavering in opinion, doubting, doubtful," from duo "two" (from PIE root *dwo- "two"), with a sense of "of two minds, undecided between two things." Old English also used tweo "two" to mean "doubt." Compare doubt (v.). Related: Dubiously; dubiousness.

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indubious (adj.)
"certain, not doubtful," 1620s, from Latin indubius "not doubtful," from in- "not, opposite of" (see in- (1)) + dubius "vacillating, fluctuating," figuratively "wavering in opinion, doubting" (see dubious). Related: Indubiously.
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dubitable (adj.)

"liable to be doubted," 1620s, from French dubitable, from Latin dubitabilis "doubtful," from dubitare "hesitate, doubt" (see doubt (v.)). Related: Dubitably.

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frill (n.)
"wavy ornamental edging," 1801 (with a doubtful attestation from 1590s), of uncertain origin despite much speculation [see OED]; figurative sense of "useless ornament" first recorded 1893. Related: Frills.
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apocryphal (adj.)
1580s, "of doubtful authenticity," from apocrypha + -al (1). Middle English had apocrive (late 14c.) in same sense. Related: Apocryphally.
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questionable (adj.)

1580s, "that may be interrogated" (a sense now obsolete); c. 1600, of facts, claims, etc., "open to dispute, doubtful," from question (v.) + -able. Deprecatory sense of "dubious in character" is attested from 1806. Related: Questionably.

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ambiguous (adj.)

"of doubtful or uncertain nature, open to various interpretations," 1520s, from Latin ambiguus "having double meaning, shifting, changeable, doubtful," adjective derived from ambigere "to dispute about, contend, debate," literally "to wander, go about, go around," figuratively "hesitate, waver, be in doubt," from ambi- "about" (from PIE root *ambhi- "around") + agere "drive, lead, act" (from PIE root *ag- "to drive, draw out or forth, move"). First attested in Sir Thomas More (1528); related ambiguity dates to c. 1400. Related: Ambiguously; ambiguousness.

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indubitable (adj.)
mid-15c., "too plain to admit of doubt," from Latin indubitabilis "that cannot be doubted," from in- "not, opposite of" (see in- (1)) + dubitabilis "doubtful," from dubitare "hesitate, doubt" (see doubt (v.)).
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wattle (n.2)
"fleshy appendage below the neck of certain birds," 1510s (extended jocularly to human beings, 1560s), of uncertain origin and of doubtful relationship to wattle (n.1). Related: Wattled.
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