1540s, from Latin dubiosus "doubtful," from 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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