To take on or accept responsibility together with another person.
From 'co-' (together) and 'assume' (to take on), from Latin 'assumere' meaning 'to take up.' Used in legal and formal contexts for joint assumption of duty or responsibility.
In contracts, when both parties 'coassume' a debt or obligation, they're both equally responsible—which is very different from one person guaranteeing another's debt, a distinction that matters hugely in court!
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