An extremely archaic legal or technical term of uncertain meaning, possibly relating to something external or foreign.
From Latin 'foris' (outside, foreign) combined with a suffix or combining form. Possibly used in medieval English legal vocabulary but very rarely attested in surviving texts.
Legal Latin words like 'forinsec' (if that's really what it meant) show how English lawyers literally borrowed Latin wholesale—sometimes a word appeared once in one document and then vanished forever.
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