Maybe a portmanteau of "sack lunch" and "regret", although I don't see how that could possibly be pronounced

There should be a word for when you pack a lunch, and it's a perfectly serviceable lunch, but then about half an hour before you eat it you catch a whiff of someone else's lunch and it gets all your juices flowing for whatever it was they brought, and you don't have an appetite for your own lunch anymore. English: It's totally inadequate!