The Dad Came Back With the Milf

Listen here, genius. The joke is that the person's dad didn't come back with milk, like a lot of dads who leave for milk and never come back. Instead, he came back with a "milf", which is slang for "Mom I'd Like to F*ck", meaning he shacked up with a hot mom instead of getting milk. You probably don't know anything about parenting because you have trouble taking care of yourself, smoothbrain.