SOUPS-on (also spelled soupçon) from the French word for "suspicion."
A suspicion or suggestion, and, by extension, a very small quantity.
"I am certain," she said, without a soupcon of doubt entering her voice.
It's most commonly used in cooking to refer to a pinch, a drop, a smidgen, or a dash of something that's just large enough to be detectable:
There's just a soupcon of lemon in these delicious butter cookies.
In either case, the word sounds a bit pretentious in modern English; it's the sort of thing you'd expect to hear from Martha Stewart or Julia Child.