Old English feola, fela (West Saxon), feolo, feolu (Mercian, Northumbrian), "much, many, in large amounts, very," from a common Germanic adjective from Proto-Germanic *felu (source also of Old Saxon filo, Dutch veel, German viel, Old Norse fiol, Gothic filu), from PIE root *pele- (1) "to fill." Obsolete; OED's last entry for it is Hakluyt (1598).
Hence felefold "manifold," from late Old English felefeald.
It was fouler bi felefold þan it firste semed. ["Piers Plowman," c. 1378]