In the manner of a grate; like a grating; with a harsh, scraping quality.
From 'grate' plus the adverbial suffix '-wise' (in the manner of, direction of). The '-wise' suffix derives from Old English 'wīse' (manner, fashion).
The suffix '-wise' is ancient (think 'clockwise'), but we've mostly stopped using it to form new adverbs in modern English, making words like 'gratewise' feel archaic even though they follow perfectly logical rules.
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