See also Scale (Fish)
SCALES, FISH (קַשְׂקֶ֫שֶׂת, H7989, LXX λεπίς, G3318, meaning fish scales). The rough exterior surface of a fish, removable by scraping.
Only those fish that had scales and dorsal fins were permissible for the OT Jew to eat. “Everything in the waters that has fins and scales, whether in the seas or in the rivers, you may eat” (Lev 11:9).
A fig. use is indicated in 1 Samuel 17:5, 38 where scales are compared to a coat of mail, and in Ezekiel 29:4, where the prophet predicts that even fish that cling to the hoary scales of a crocodile share his fate when it is caught, so also would the followers of the arrogant Egyp. pharaoh share the calamity that would overtake their leader.