This is the 2603rd most frequent German word.
außer
"Außer" means "except" or "besides."
In this sentence, 'außer' is used as a preposition meaning 'except', excluding 'fish' from the set of things the speaker eats.
Ich esse alles außer Fisch.
I eat everything except fish.
Here, 'außer' introduces a phrase that specifies an exception among the subjects, distinguishing the cat from the other animals.
Außer der Katze sind alle Tiere draußen.
Except for the cat, all animals are outside.
This usage of 'außer' demonstrates its role in emphasizing the exclusivity of 'the truth' in what he speaks.