means One of two children born from the same pregnancy, or, more loosely, either of two things that match each other exactly.

from From Old English 'getwinn,' meaning a double or one of two born together, rooted in the ancient sense of 'two.' It belongs to a wide Germanic family — related to Old Norse 'tvinnr' (double) and a cousin of words like 'twine' (two threads twisted into one) and 'between.' The thread running through them all is the number two, doubled and intertwined.