Idiom for making up a false story as an excuse to get out of something

Solution 1:

Not an idiom, "pretext" seems to fit

A reason or excuse given to hide the real reason for something, a fictitious reason given in order to conceal the real one. TFD

It was a pretext for not joining us.

Solution 2:

This answer is from the point of view of the person to whom the excuse is given: fairy tale

John told me another fairy tale about why he can't finish repairing the car until next week.

fairy tale from Oxford Dictionaries

A fabricated story, especially one intended to deceive

Solution 3:

An idiomatic expression is to tell a cock-and-bull story :

• a story or explanation which is obviously not true.

  • She told me some cock-and-bull story about her car breaking down.

(Cambridge Idioms Dictionary,)

Solution 4:

Here are a few:

fob someone off [with some excuse]

brush someone off [with some excuse]