redoubtable adj.

c. 1200, douten, duten, "to dread, fear, be afraid" (a sense now obsolete), from Old French doter "doubt, be doubtful; be afraid," from Latin dubitare "to doubt, question, hesitate, waver in opinion" (related to dubius "uncertain"), from duo "two" (from PIE root *dwo- "two"), with a sense of "of two minds, undecided between two things." Compare dubious. Etymologically, "to have to choose between two things."

The sense of "fear" developed in Old French and was passed on to English. Meaning "to be uncertain, hesitate or waver in opinion" is attested in English from c. 1300. The transitive senses of "be uncertain as to the truth or fact of" and "distrust, be uncertain with regard to" are from c. 1300.

The -b- was restored 14c.-16c. in French and English by scribes in imitation of Latin. French dropped it again in 17c., but English has retained it.

It replaced Old English tweogan (noun twynung), from tweon "two," on notion of "of two minds" or the choice between two implied in Latin dubitare. Compare German Zweifel "doubt," from zwei "two."

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."

also redout, "small, enclosed military work," c. 1600, from French redoute (17c.), from Italian ridotto, earlier ridotta, "place of retreat," from Medieval Latin reductus "place of refuge, retreat," noun use of past participle of reducere "to lead or bring back" (see reduce). The unetymological -b- was added by influence of unrelated and now obsolete English verb redoubt "to dread, fear" (see redoubtable). As an adjective, Latin reductus meant "withdrawn, retired; remote, distant."