-fold

multiplicative word-forming element attached to numerals, from Old English -feald, Northumbrian -fald, from Proto-Germanic *-falda- (cognates: Old Saxon -fald, Old Frisian -fald, Old Norse -faldr, Dutch -voud, German -falt, Gothic falþs), combining form of *falthan, from PIE *polt-, extended form of root *pel- (2) "to fold."

The same root yielded fold (v.) and perhaps also Greek -ploid, -plos and Latin -plus (see -plus). Native words with it have been crowded out by Latinate double, triple, etc., but it persists in manifold, hundredfold, etc.