twin (adj.) Look up twin at Dictionary.com
Old English twinn "consisting of two, twofold, double," probably ultimately from Proto-Germanic *twinjaz (cf. Old Norse tvinnr, Old Danish tvinling, Dutch tweeling, German zwillung), from PIE *dwisno- (cf. Latin bini "two each," Lithuanian dvynu "twins"), from *dwi- "double," from root *dwo- "two" (see two). The verb meaning "to combine two things closely" is recorded from late 14c. The noun developed from Old English getwinn "double."