Why is the ninth month called September?
Old English monigfald (Anglian), manigfeald (West Saxon), "various, varied in appearance, complicated; numerous, abundant," from manig (see many) + -feald (see -fold). A common Germanic compound (cf. Old Frisian manichfald, Middle Dutch menichvout, German mannigfalt, Swedish mångfalt, Gothic managfalþs), perhaps a loan-translation of Latin multiplex (see multiply). Retains the original pronunciation of many. Old English also had a verbal form, manigfealdian "to multiply, abound, increase, extend."
Old English manigfealdlic "in various ways, manifoldly," from the source of manifold (adj.).
in mechanical sense, first as "pipe or chamber with several outlets," 1884, see manifold (adj.); originally as manifold pipe (1857), with reference to a type of musical instrument mentioned in the Old Testament.