Origins
The word 'twin' is, at its etymological core, simply the word 'two' with a suffix. It descends from Old English 'getwinn,' meaning 'double' or 'twofold,' which in turn derives from Proto-Germanic *twinjaz, built on the PIE numeral root *dwóh₁ meaning 'two.' Every time we say 'twin,' we are saying a decorated version of 'two.'
The Old English form 'getwinn' was both an adjective meaning 'double' and a noun meaning 'one of a pair born together.' The prefix 'ge-' was a common Old English collectivizing element (compare 'gebrōþor,' meaning 'brothers collectively'). When this prefix was lost in Middle English, the word simplified to 'twin' or 'twinne.'
Across the Germanic languages, the root produced parallel formations. Old Norse 'tvinnr' meant 'double' or 'twin,' and modern Icelandic retains 'tvíburar' (twins). German 'Zwilling' (twin) shows the same root with a different suffix: 'Zw-' from Proto-Germanic *tw- (the 't' to 'z' shift is regular in High German via Grimm's Law and the High German consonant shift), plus the diminutive suffix '-ling.' Dutch 'tweeling' follows the same pattern.
Proto-Indo-European Roots
The PIE root *dwóh₁ is one of the most securely reconstructed words in comparative linguistics, with reflexes in virtually every Indo-European language. It gave Latin 'duo,' Greek 'dúo,' Sanskrit 'dvau,' Old Irish 'dá,' Lithuanian 'du,' and Old Church Slavonic 'dŭva.' In the Germanic branch, regular sound changes (Grimm's Law converting *d to *t) produced Proto-Germanic *twō, the ancestor of English 'two.'
The family of English words built on this 'two' root is remarkably large. 'Two' itself is the direct descendant. 'Twelve' comes from Old English 'twelf,' literally 'two left' (that is, two remaining after ten). 'Twenty' is from 'twentig,' meaning 'two tens.' 'Twice' is 'two times.' 'Twine' — cord made by twisting two or more strands together — derives from the same concept of doubleness. 'Twist' itself may be related. And 'between' contains the same root, from Old English 'betwēonum,' literally 'by twos' or 'in the space of two.'
Mythologically, twins occupy a special place across Indo-European cultures, and the linguistic evidence supports this. The Greek Dioscuri (Castor and Pollux), the Roman Romulus and Remus, the Vedic Ashvins, and the Norse tale of Ask and Embla all feature paired figures. The PIE speakers may have had a twin myth of their own, given how widespread the motif is among their linguistic descendants.
Old English Period
The word 'twin' itself has extended metaphorically throughout its history. 'Twin cities' (like Minneapolis and St. Paul) dates from the nineteenth century. 'Twin' as a verb meaning 'to pair' or 'to match' appeared in Middle English. The adjective 'twin' (as in 'twin beds' or 'twin engines') preserves the original Old English adjectival sense of 'double' or 'twofold.'
In modern genetics, the distinction between 'identical twins' (monozygotic, from one fertilized egg) and 'fraternal twins' (dizygotic, from two eggs) has given the word new scientific precision. But the etymological core remains unchanged: 'twin' still means what it meant in Old English — two of a kind, born together, double.