“There was nothing to fear and nothing to doubt…” - Radiohead, “Pyramid Song” (from the album Amnesiac)
In these uncertain and doubtful times, well, I thought it would be helpful to go through the etymology of the word “doubt.” Now, of course, before it makes its way to English, it goes through a lot of intermediary languages, but my concern is generally with penetrating back to the source. “Doubt” comes from Latin dubitare, which means “to be uncertain, to be in doubt, to waver in opinion or judgment” (L&S, i.e., Lewis & Short’s A Latin Dictionary, s.v. dubito). More directly from dubitare, we get things like “dubious” (= “doubtful”), “indubitable” (= “undoubtable”), “indubitably,” “dubitation” (though that’s admittedly rather archaic; lit. “the state of doubting, the action of doubting”). And of course, once it morphs to “doubt” (through the French, which lengthens the Latin long -u- to the diphthong -ou), we get “undoubtedly,” “doubtful,” and I’m sure there are others I’m missing. English “redoubt” is not related, though; it comes, via French, from Latin reducere, “to withdraw, draw back,” meaning “a place of refuge” (= a place to which one withdraws).