'dubious' originates from Latin, specifically the word 'dubius', where 'du-' (from 'duo' / uncertainty) and the root meant 'doubtful' or 'wavering'.

'dubius' passed into Late Latin and then into Middle English as 'dubious', retaining the same basic form and sense; the adverbial form 'dubiously' was formed in English by adding the adverbial suffix '-ly'.

Initially it meant 'doubtful, wavering', and over time it has remained close to that sense, now used both for uncertainty of belief and for suspiciousness.