Merriam-Webster Collegiate Dictionary - in-

 I. prefix  or il- or im- or ir-  Etymology: Middle English, from Anglo-French, from Latin; akin to Old English un- not ; non-, un- — usually il- before l illogical, im- before b, m, or p imbalance immoral impractical, ir- before r irreducible, and ~ before other sounds inconclusive  II. prefix  or il- or im- or ir-  Etymology: Middle English, from Anglo-French, from Latin, from in in, into  1. in ; within ; into ; toward ; on — usually il- before l illuviation, im- before b, m, or p immingle, ir- before r irradiance, and ~ before other sounds infiltrate  2. en- I imbrute imperil inspirit