"top notch" means "first class", "among the best ", "a-one".
Example: Our school has a top notch baseball team.
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A top notch pilot is referred to as an ace.
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c. 1577 from 'otch', from Middle French 'oche' (notch) Old
French 'ochier' " to notch". Top notch would indicate an arrow
notch that could be brought to the bowstring and 'notched' quickly,
increasing bowman's rate of fire