A person or thing that causes grime or makes something grimy and dirty.
From begrime (verb) + -er (agent suffix meaning 'one who does'). The -er suffix creates a noun identifying the cause or agent of the action.
Agent nouns with '-er' are one of English's most productive word patterns—basically any verb can theoretically become a noun describing what does the action. So a 'begrimer' would be that factory smokestack or grimy coal truck that makes everything dirty.
Complete word intelligence in one call. Free tier — 50 lookups/day.