The highest points or upper parts of something; or informal slang meaning excellent or the best.
From Old English 'top,' originally meaning 'a tuft of hair' or 'the highest point.' The slang meaning 'excellent' emerged in early 20th-century American English.
That something excellent is 'tops' shows how we borrow from physical descriptions—the 'top' of a list is best, so calling something 'tops' means it's on top of the ranking!
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