Plural of grindstone; large rotating stones used to sharpen tools or grind grain, or figuratively, hard work or burden.
Compound of 'grind' and 'stone'; Old English 'grindan' (grind) plus 'stan' (stone). Grindstones have been used since ancient times for sharpening and grinding.
The phrase 'nose to the grindstone' comes from actual medieval workers—spending hours with your face near a grindstone to see your tool's sharp edge, which made the metaphor for constant hard work perfectly literal and painful.
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