An archaic term possibly meaning to heed thoroughly or to heed against; may relate to warning or paying attention.
From 'for-' (intensive or sometimes negative prefix in Old English) combined with 'heed' (to pay attention to, from Old English 'hēdan'). The 'for-' prefix may have shifted the meaning toward protection or forewarning.
If 'forheed' meant 'to heed against,' it's the linguistic ancestor of the modern idea of a 'heads up'—warning someone to pay attention to an approaching danger.
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