What is the antonym of necktie?

There isn't really a direct antonym for "necktie". It's a specific item, and there isn't a word that perfectly captures the opposite of wearing a necktie.

However, depending on the context, you could use some words that convey the opposite meaning:

* Informal: "casual", "open collar", "no tie", "untidy"

* Formal: "bow tie" (though still a type of tie)

For example:

* "He wore a necktie to the interview, but he prefers to dress more casually for work."

* "The dress code requires a necktie, but you can choose between a regular tie or a bow tie."

Ultimately, the best word to use depends on the specific situation and the meaning you want to convey.