Are you asking when we use 'silly' and when we use 'fool'? ('Foo' doesn't exist.) The first is an adjective: 'You're silly' , while the second is a noun: 'You're a fool'. The adjective from 'fool' is 'foolish'. Silly and foolish are similar in meaning, but 'foolish' is often more extreme. 'Silly' means childish, while 'foolish' is closer to idiotic.

I think you mean" silly old fool" it is a way to tell someone that they are not clever or foolish.