"He is healthy as a fish!"
Translation:Han er frisk som en fisk!
We don't really say "syk som en" anything. There are lots of expressions with animals, but I can't think of any describing illness. Try "slu som en rev" (devious as a fox), "dum som en stut" (dumb as a bull) or "å gjøre noen en bjørnetjeneste" (to do someone a bear favour - i. e. to mean well trying to help someone, but ending up hurting them instead)
Thanks so much! Those are all good to know but I am especially delighted by "å gjøre noen en bjørnetjeneste." Gotta figure out how to work it into a conversation.
We say someone is sly as a fox or dumb as an ox. You can also be "crazy like a fox," an expression that goes back not quite a century and means what you might expect: to act so crazy that people underestimate you and you can take advantage of them.
Anyway, I can now add a few more items to my Norwegian collection of "Cliches 'r Us." :) Tusen takk!