An archaic or dialectal form meaning to chat or converse casually, or possibly an alternate form of 'cozen' (to deceive or trick).
Either from the archaic verb 'cose' (to chat), or a variant of 'cozen' from uncertain origin, possibly related to confidence tricks and deception.
The ambiguity of 'cosen' is fascinating—it might come from casual conversation that's so comfortable it borders on conspiratorial, or it might be a deceptive form of friendly talk, showing how closely conversation and trickery can be linked.
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