Oh, I know, you’re confident on this one. Just listen to that pronunciation!
Starting off with refreshers, we know that a tasty post-meal treat is a “dessert” (s x 2). We know that cacti grow in “deserts” (s x 1).
When you use the “just deserts”/”just desserts” expression, it is pronounced the same way you say “desserts.” It’s clearly not talking about a sandy, dry locale. There’s probably some etymological backstory about a poisoned soufflé or something, right? Yeah, that guy got his “just desserts,” someone once said, and it stuck, right? Someone like Shakespeare? Something from Titus Andronicus? If you think about it hard enough, it almost makes sense, doesn’t it?
But then, of course, you would be wrong.
The expression “just deserts” (yes, one “s”) has the same root as the word “deserve.” The original Latin form was deservire, which meant to serve well or to serve zealously. This is different from the Latin word deserere, which means to forsake. The latter is the origin of the typical arid “desert” we think about.
Today, both “just deserts” (the correct form) and “just desserts” (the commonly assumed correct form) are used quite frequently, but now you know which one is correct. And wouldn’t you rather be using your words accurately?