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.-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.