Source : Webster's Revised Unabridged Dictionary (1913)

In- \In-\ [See {In}, prep. Cf. {Em-}, {En-}.]
   A prefix from Eng. prep. in, also from Lat. prep. in, meaning
   in, into, on, among; as, inbred, inborn, inroad; incline,
   inject, intrude. In words from the Latin, in- regularly
   becomes il- before l, ir- before r, and im- before a labial;
   as, illusion, irruption, imblue, immigrate, impart. In- is
   sometimes used with an simple intensive force.

In- \In-\ [L. in-; akin to E. un-. See {Un-}.]
   An inseparable prefix, or particle, meaning not, non-, un-
   as, inactive, incapable, inapt. In- regularly becomes il-
   before l, ir- before r, and im- before a labial.