To make strong or to strengthen something.
From 'en-' combined with 'strengthen', itself built from 'strength' plus '-en'. This is a doubly-affixed verb showing the Middle English love of intensification—adding 'en-' to an already-strengthened verb.
Medieval scribes loved the redundancy of this word—it shows how writers piled on prefixes to show extreme intensity, much like how we today might say 'I very strongly believe' instead of just 'I believe', except they did it with word formation itself.
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