User blog:Mgf60/Vol.1 Ch.41 Naver Endnote translation explanation

Original Korean: 월척이로구나!!

This word can be split into three parts

월척 - The act of catching a big fish (Think around 1.5 meters) or the big fish itself. Can be used as "What a catch".

이 - Subject particle

로구나 - A sentence-final ending used to imply a certain feeling in a newly learned fact. Given that the newly learned fact is that the writer caught a big fish combined with the use of two exclimation marks, the feeling that is intended to be conveyed seem to be excitement.

The whole word is therefore meant to describe excitement at the fact that the writer managed to catch a big fish.

A simple way of translating this would be "What a catch!!".