According to Wikipedia: "The original sense of the word is of a 'plug' or 'wad' used to fill a hole ... derived from Mexican Spanish 'light lunch'."
We'll keep it paleo ...
First, the fish. Use some firm white fish: cod is popular; I like pollock.
You could batter the fish and shallow fry, the batter made from egg and some kind of starch, or simply roll the fish in starch. I did the latter, simply dusting in cocoyam flour and shallow frying in beef dripping for a couple of minutes each side. Done. How easy is that?
Back up ... cocoyam flour? Yup! Any starch will do: yam, fufu, potato, cornstarch, at a push, even arrowroot. Just something to dust and to take on some colour while frying in the dripping.
Accompany with a salad, or Pico de Gallo, large pieces of tomato, cucumber, avocado, pepper and onion; some Greek-style yoghurt or sour cream, your favourite chilli sauces and a few wedges of lime.
The Taco? Easy! Gem lettuce.