The most convenient, useful, or easy to handle or use; the superlative form of handy.
Superlative form of 'handy' using the '-est' suffix, from Old English 'hændig' with roots in Germanic languages where '-est' marks the highest degree of an adjective.
The fact that 'handiest' is used for both 'most skillful person' and 'most convenient tool' shows that usefulness and skill were historically inseparable—if you could use something with your hands quickly, it was valuable.
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