making something strong even stronger; adding fins to a lion
Rare idiomatic expression meaning to add extra power or advantage to something already formidable. The literal image is giving fins to a lion, which is already a powerful creature.
The saying 'shishi ni hire' means adding even more strength to something already strong.
The exact origin is uncertain. It is a classical Japanese idiom combining 獅子 (lion) and 鰭 (fins), implying an already powerful creature receiving an additional advantage.