I'm not really sure what you mean. Looking at the sample sentences at jisho.org which are Tanaka/Tatoeba, they all fit roughly with that definition.
まさか is an exclamation, so it's going to be hard to provide an exactly translation that explains how it's used, but I take it that it is used for surprising information.
Looking at the first example:
I might translate it depending on the level of formality you want the English to have as any of the following:
(1) Wow, you've got to be kidding!
(2) No way, that's a lie right?
(3) That's unbelievable. It's a lie right?
I haven't really seen it used this way much which is also in the corpus (but that could just be a limit of who I encounter and when):
In these まさかの時 constructions, I take the meaning to be "unbelievable/unlikely times"
= He insures himself for unlikely times.