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."
twinning twin·ning (twĭn'ĭng)
The bearing of twins.
A pairing or union of two similar or identical objects.
One of two offspring born at the same birth. adj.
Being two or one of two offspring born at the same birth.
Consisting of two identical or similar parts; double.