Dainoji?

Literally, "the 'dai' kanji", which generally refers to sprawling with one's limbs out so as to form the character 大.

FJH said:

Dainoji?

Moonspeaker said:

Literally, "the 'dai' kanji", which generally refers to sprawling with one's limbs out so as to form the character 大.

Using the first kanji of someone's name and then adding "noji" to it is also a simple way of creating a nickname in Japan, from what I've heard, so "Dainoji" seems to be an extremely simple nickname for "Daiyousei".

Alice_Komeiji said:

Using the first kanji of someone's name and then adding "noji" to it is also a simple way of creating a nickname in Japan, from what I've heard, so "Dainoji" seems to be an extremely simple nickname for "Daiyousei".

Kinda. It was in vogue during the Edo period among red-light district workers, yakuza (precursors), and other 'low-class' types. It's not a way of forming nicknames so much as it's an euphemistic expression used to refer to someone else (in a circuitous way), so that only people 'in the know' would understand who the speaker is talking about. It's like calling someone Mr. S or Miss A. So yeah, you use it to gossip about people and to talk about shady business, making it the 'medieval' Japanese equivalent of Thieves' Cant.

This 'slang' is likely derived from Muromachi-era courtlady cant (Nyōbō-kotoba), where it was used for court gossips.

In modern times, this expression is very dated; you'll likely only hear it in jidai-geki period films and among Yakuza (where again it functions like Thieves' Cant). And even among Yakuza it's falling into disuse. Granted, there is a resurgence of sorts recently in some newer fictional works set in jidai-geki periods (or Western fantasy where it's used as an equivalent) written by younger writers (especially the web novel kind) who didn't understand how it's used (likely they just heard someone use the word in an older jidai-geki film and mistakenly assumed it's a generally purpose nickname modifier) and just slap it nilly-willy in their writings. (Like Western writers spamming thous and thees and -th/st endings where they don't belong.)

The modern equivalent is to use the initial Latin letter of a person's romanized name + さん (or some other 'honorific'). Like Ａさん、Ｋくん、Ｙちゃん. (Again, the equivalent of Mr. A, Ms. Y, etc.) This is used for gossiping or to protect a person's identity when discussing about confidential matters. And again, like ～の字 this doesn't usually work as a nickname because you (usually) don't use it to directly refer to somebody (in the 2nd person). Of course, exceptions do exist, like how Mr. T works for Laurence Tureaud in America.