noun
what name; what's-his-name
Used to refer to a young male whose name the speaker cannot recall or does not know, often with a slightly dismissive or casual tone. The suffix 君 is typically used for boys or young men.
あの何君って言ったっけ、また遅刻だよ。
What's-his-name, that guy, is late again.
クラスに何君か忘れたけど、背の高い子がいるんだ。
There's a tall kid in class, though I forget his name.
誰 is the general question word for 'who', while 何君 specifically asks for or refers to a young male's name with the suffix 君.
何ちゃん is the equivalent for young girls or children, using the diminutive suffix ちゃん instead of 君.
Compound of 何 (nani, 'what') and the name suffix 君 (kun), used as a placeholder for a young male's name.