Domestic cats do not smack their lips. They use their tongue to groom themselves around the mouth because they are fastidiously clean creatures. They need to remove food particles from around their mouth; called ‘autogrooming’. As opposed to ‘allogrooming’ which is grooming another cat or a person. It’s a simple as that really. The phrase “to smack your lips” applies to humans and it usually means that a person is looking forward to the food that is going to be served up. Although it also applies to other enjoyable activities. People indicate this by opening and closing their mouth and perhaps verbalising their feelings.
The phrase “licking one’s lips” means that a person is excited about what is to come; a very similar meaning. It does not necessarily mean that they actually lick their lips, which, by the way, is exactly what a domestic cat does (and around the mouth) after they have eaten their meal. And after licking their lips they may go into a full programme of self-grooming, following a set pattern which does not change.
RELATED: How Does Your Cat Groom Herself?
When a cat licks the side of their mouth to self-groom, this should be differentiated from nose-licking which is a displacement activity brought about because of uncertainty. The picture shows the difference.
One last point: cats also lick their lips in a cursory way when a little uncertain. It is a variation on displacement activity. To complicate matters more, there is an overlap between nose licking displacement activity and grooming around the mouth.
Below are some more articles on grooming.