"C'est malin" means that's cunning or crafty.

I dissagree with Shachi and Nawel. The words themself do mean "cunning or malicious" but the French use them out of context, to ponctuate their speach. It sort of mean "nice work" but it's always said ironically. Let's look at one example : - J'ai oublié d'apporter le livre que tu m'a demandé. - Ha c'est malin ! So really it means "how annoying!/ how stupid!/ look at what's you`ve done/ what are we going to do about this now?"

literally means "It's smart!" but often used in ironic form

its malicious