prefix meaning "into, in, on, upon" (also im-; il-; ir-
by assimilation of -n-
with following consonant), from L. in-
"in" (see in
). In O.Fr. this often became en-,
which was usually respelled in Eng. to conform with L., but not always, which accounts for pairs like enquire/inquire.
There was a native form, which in W.Saxon usually appeared as on-
(cf. O.E. onliehtan
"to enlighten"), and some verbs survived into M.E. (cf. inwrite
"to inscribe"), but all now seem to be extinct.