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" (see 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.