Full of doubt; characterized by uncertainty or suspicion (archaic form of doubtful).
From 'doubt' plus the archaic adjectival suffix '-ous' (from Latin 'osus,' meaning 'full of'). This formation was common in Middle English and Early Modern English (seen in words like 'wondrous' and 'monstrous'), but 'doubtful' eventually became the standard form.
Medieval writers loved '-ous' endings for emotional states—they'd write 'doubtous,' 'fearous,' and 'wrathous'—but over time English speakers simplified these to '-ful' forms, which is why we say 'doubtful' today instead of 'doubtous,' even though both are technically correct.
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