It is often asked: What does the word Maru mean in the names of ships-as Tōkyō Maru, Sagami Maru, Hiryū Maru, etc.? The answer is that the origin of the term is obscure. Maru means "round;" but how came ships by so inappropriate a name?
The first thing to note is that in former times ships had not the monopoly of the name. Swords, musical instruments of various kinds, pieces of armour, dogs, hawks, and the concentric sections of castles, were called Maru also. The probability is that two distinct words-maru and maro-have flowed into one, and so got confused. To name the concentric sections of a castle maru, "round," was but natural. The word maro, on the other hand, is an archaic term of endearment. Hence its use in such ancient proper names as Tamura-Maro, a great general who subdued the Ainos; Abe-no-Nakamaro, an eminent scholar of the eighth century; Okina-Maro, a favourite dog of the Emperor Ichijō, and so on. The warrior's pet sword, the sportsman's favourite dog or hawk, the oarsman's boat, would naturally come to be distinguished by the same half-personal name, much as the English sailor or engineer calls his ship or locomotive "she." When the ancient word maro ceased to be understood, it would easily slide into the more familiar maru, by the alteration of the final vowel, o and u being particularly apt to interchange in Japanese.
Observe that Maru is applied to merchant vessels only. Menof-war take Kan instead, as Maya Kan, Asama Kan. Kan was originally a Chinese word meaning "war-vessel." It is now pronounced lan in China itself, and is no longer there used in the same sense.