The phrase "what is his" is an informal way of referring to a person whose name you either don’t know, can’t remember, or choose not to mention. It’s often used in spoken English as a placeholder, similar to “so-and-so” or “you-know-who.”
For example: “I was talking to what’s-his-name from accounting yesterday.” In this sentence, the speaker doesn’t recall or doesn’t want to state the actual name of the person from the accounting department.
Note that the more common form is actually “what’s-his-name” (with an apostrophe and hyphen), especially when used as a noun phrase. “What is his?” without context might sound like a literal question about possession—for instance, asking “What belongs to him?”—but that usage is rare in modern conversation.
This expression has been part of colloquial English for over a century and reflects how speakers manage gaps in memory or social discretion in everyday talk.