I was watching a movie called The book of Henry. One day Henry's mother goes to work late. Her manager told her, "Late again!", and she replied, "Dog ate my car." So, I heard about Dog ate my homework, but I have never heard about that excuse.
So, I heard about Dog ate my homework, but I have never heard about that excuse.
That's the joke exactly. "The dog ate my homework" is an implausible excuse. Everyone knows that it's probably a lie, and that most likely the student never did their homework, but doesn't want to say so.
"The dog ate my car" is an absurdly implausible excuse. It means "I don't have a good reason for being late, or I'm not willing to give you one". But telling that to your boss is rude and confrontational in a way that telling a completely unbelievable lie, strangely, is not.
'[The] Dog ate my (whatever)' is a generic lame excuse, another way of saying 'I have no excuse'. I was late to be married because the dog ate my wedding suit (really I was drunk the night before and overslept). Maybe the boss doesn't really care about her being late, and it's a joke between them. Maybe not. I have not seen the film.
Dog ate my homework is a typical excuse used for someone not doing their homework.
In this case “dog ate my car” it is just a joke as obviously a dog cannot eat someone’s car.
In other words the person was most likely late for something. Or couldn’t drive to a particular place and this is just a funny excuse.