Ne pas être dans son assiette, literally “to not be in one’s plate”, means to not be feeling very well. In fact, “assiette” comes from “‘asseoir” (to sit) and used to refer to the way of sitting in the 16th century. In a figurative way it refers to a state of mind and this is where the expression comes from. The closest expression in English would be “to be under the weather”.
French with Valerie 