Many dictionaries define "only too" as "very." If so, what does "only too" mean in the following?

"That was only too good to be true"

Does it mean "that was very good to be true"? Or "that could not be true at all"?

I'd appreciate your help.

Many dictionaries define "only too" as "very." If so, what does "only too" mean in the following?

"That was only too good to be true"

Does it mean "that was very good to be true"? Or "that could not be true at all"? What is the meaning of "only" here? I'd appreciate your help.