"Saba o yomu （鯖を読む）" literally means, "to read the mackerel." An insult or a complement? Read this very short article to find out.
Japan is an island nation, therefore seafood has been essential to the Japanese diet since ancient times. Although meat and dairy products are as common as fish today, fish is still the main source of protein for the Japanese. Fish can be prepared grilled, boiled, and steamed, or eaten raw as sashimi (thin slices of raw fish) and sushi. There are quite a few expressions and proverbs including fish in Japanese. I wonder if this is because fish are so closely related to Japanese culture.
Famous Japanese Fish Proverbs (thoughtco.com)