The phrase is not about eating, but suggests someone's mind is elsewhere.

Metaphorical Use

It's casual language and fits best in informal dialogue or narration.

Informal Setting

Understand it's a figure of speech for not being mentally present.

It can be a mild and humorous way to question someone's judgement.

Often used to exaggerate someone's disconnect from reality.

Be cautious when using this phrase as it can be taken as offensive.

The phrase can simply mean someone is not available, not eating.

Usually, it refers to the typical lunch break period, around midday.

It implies the person will return soon, particularly from a break.