Literally translated as, “not being in one’s plate,” ne pas être dans son assiette is a colloquial expression that means feeling under the weather, being out of sorts, physically and/or morally. You will also commonly hear the variation ne pas avoir l'air dans son assiette to describe not looking like oneself, looking like one isn't well .
Note that both idioms are mostly used in the negative, and the possessive adjective will change to agree with the subject.
Why would one want to be in one's plate in the first place? Well, you wouldn't... It turns out the expression has nothing to do with a plate!
Long before it came to be an individual serving vessel, in the sixteenth century, l’assiette was the place where one was seated at the table, or the way one was sitting. It is still used today to refer to a horse rider’s posture, the stability of a plane or boat, and more generally, something firm and stable on which other things can be built, literally or figuratively.
In this new light, it appears that “not being in your plate” really comes from the idea that you’re not sitting in your habitual seat, that you’re not feeling like you usually do, and that you’ve lost some of your balance and stability.
Je ne suis pas dans mon assiette ce matin
I don’t feel well this morning.
Ça va ? Tu n’as pas l’air dans ton assiette aujourd’hui.
Are you ok? You don’t look yourself today.