What does postprandial mean?

Would you accept a request for a "postprandial constitution" from a nervous date on your first evening out? You might say yes — if you liked him (or her) enough and knew he was asking for an after-dinner walk with you! Postprandial is a fancy way of saying "after a meal," and the word is used especially to mean "after dinner."

Here's a scene from Fyodor Dostoevsky's The Idiot,

[The general] did not finish his sentence, for at this moment Ferdishenko pushed a chair up from behind, and the general, not very firm on his legs, at this postprandial hour, flopped into it backwards. It was always a difficult thing to put this warrior to confusion, and his sudden descent left him as composed as before.