The plural of two; multiple instances of the number 2, or the phrase 'in twos' meaning in pairs.
From Old English 'twa,' from Proto-Germanic 'twai.' The word is among the oldest in English, appearing in some form in virtually all Indo-European languages.
The number two is unique in English because it's one of the only numbers that changes form entirely—we say 'two' but 'second,' while three becomes 'third'—a remnant of ancient grammar that linguists call 'suppletive forms.'
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