What does funny as a rubber crutch mean?

The phrase "funny as a rubber crutch" is an idiomatic expression that implies something is not funny at all, often used to describe a joke or situation that falls flat. Rubber crutches are inherently impractical and ineffective, making the comparison suggest that the humor is equally lacking or absurd. Essentially, it's a way to convey that something intended to be humorous is actually quite the opposite.