To sign a document or loan agreement jointly with another person, sharing responsibility.
From Latin 'co-' (together) + 'sign' (from Latin 'signum' mark), literally meaning to 'mark together'.
When you cosign for someone's loan, you're legally promising to pay the debt if they don't—it's why banks ask for cosigners and why it's so risky; it's been called 'friendship on trial' because money problems can destroy relationships.
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