Etymology
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Words related to doubt

*dwo- 

Proto-Indo-European root meaning "two."

It forms all or part of: anadiplosis; balance; barouche; between; betwixt; bezel; bi-; binary; bis-; biscuit; combination; combine; deuce; deuterium; Deuteronomy; di- (1) "two, double, twice;" dia-; dichotomy; digraph; dimity; diode; diphthong; diploid; diploma; diplomacy; diplomat; diplomatic; diplodocus; double; doublet; doubloon; doubt; dozen; dual; dubious; duet; duo; duodecimal; duplex; duplicate; duplicity; dyad; epididymis; hendiadys; pinochle; praseodymium; redoubtable; twain; twelfth; twelve; twenty; twi-; twice; twig; twilight; twill; twin; twine; twist; 'twixt; two; twofold; zwieback.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dvau, Avestan dva, Greek duo, Latin duo, Old Welsh dou, Lithuanian dvi, Old Church Slavonic duva, Old English twa, twegen, German zwei, Gothic twai "two;" first element in Hittite ta-ugash "two years old."

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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.

dubitable (adj.)

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

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.)).
misdoubt (v.)

1530s, "to have doubts (of the reality of), to suspect, to regard (the truth or reality of) with suspicion," from mis- (1) "badly, wrongly" + doubt (v.). Meaning "to fear or suspect (the existence of something evil) is from 1560s. Intransitive sense of "entertain doubt" is from 1630s. Related: Misdoubted; misdoubting. As a noun, "irresolution," 1590s.

redoubtable (adj.)

late 14c., of persons, "worthy of honor, venerable" (a sense now obsolete); late 15c., "that is to be dreaded or feared, formidable, terrible," also often "valiant," from Old French redoutable (12c.), from redouter "to dread," from re-, intensive prefix, + douter "be afraid of" (see doubt (v.)).

The verb also was in Middle English, redouten, "to fear, dread; stand in awe or apprehension of; honor" (late 14c., from Old French) and was used through 19c., though OED marks it "now rhetorical."

undoubtable (adj.)
early 15c., from un- (1) "not" + doubt (v.) + -able. Related: Undoubtably.
undoubted (adj.)
mid-15c., from un- (1) "not" + past participle of doubt (v.). Related: Undoubtedly.
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).

doubtless (adv.)

"without doubt, without objection or uncertainty," late 14c., from doubt (n.) + -less. From late 14c. as an adjective, "beyond dispute, certain;" from mid-15c. as "free from doubt." Later in a weakened sense, indicating merely something that to the speaker seems likely to be true. Related: Doubtlessly.