This word does usually appear in dictionaries.
But be careful! We do not find it under letter מ but under letter א.
Of course this is illogical from our viewpoint... Only... this is quite logical from the Hebrew viewpoint.
Because this is a word resulting from merging of two prepositions: first, preposition מִן after having dropped its final nun; second, preposition אֶת, that is both the direct object mark and a preposition meaning with.
The word as such is to be translated by when it means the author of a given work.