Literally "Already eaten"
The experience of re-tasting the flavour of recently ingested foodstuffs...
Milder experiences are effected by a belch.
More severe instances involve retching. The impact can be heightened by chewing and re-swallowing the retched foodstuff, (see: "Cud")
"Ugh, I just had a déjà mangé."
"What was it?"
"Milk, but this time sour."