·- 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.
II. In- ·prep 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.