means The money you regularly receive, especially from work, investments, or other sources, before it goes back out the door.

from A transparent compound of 'in' plus 'come' — literally that which 'comes in.' English has used 'come in' for arrivals and yields for centuries, and by the time 'income' settled into its money-related sense, it simply meant the stuff coming into your hands. It mirrors its mournful twin 'outgo,' and shares the same plainspoken logic as 'outcome' — what comes out — making it one of those rare financial terms that means exactly what its parts say.