There are various theories on the origin of the word munagi, ranging from those that say that eels are round and slender just like a 'munagi' (ridgepole), their chests are yellow as in 'munagi' (yellow chest), and the chest is slit open upon cooking as in 'munabiraki' (opening the chest), among others.
そもそものムナギの語源には、家屋の「棟木（むなぎ）」のように丸くて細長いから、胸が黄色い「胸黄（むなぎ）」から、料理の際に胸を開く「むなびらき」から、など、いろいろな説がある。