n. An assimilated form of in- before a labial. In the following words, in the etymology, the prefix im- is usually referred directly to the original in-.n. An assimilated form (in Latin, etc.) of in- before a labial. In the following words, in the etymology, im- is usually referred directly to the original in-.n. An assimilated form (in Latin, etc.) of the negative or privative in- before a labial. In the following words, in the etymology, im- is usually referred directly to the original in-.