A small pang of insecurity when you see a partner being affectionate with a metamour — real, but usually fleeting.
Jealousy gets all the headlines; the wibble is its smaller, more honest cousin. It isn't the house-on-fire emotion — it's the flicker you feel watching a partner laugh at someone else's joke, hold a metamour's hand, text somebody good night. A pang, a wobble, gone in a few breaths. Naming it is the whole trick: "I'm having a wibble" is a sentence you can say out loud without lighting a fuse, which is exactly why the word earns its keep.
It has a bright-side twin. Where a wibble is the small ouch, a "frubble" — close kin to the better-known compersion — is the small glow you get from seeing a partner happy with someone else. Poly communities keep coining this kind of vocabulary on purpose. As the More Than Two crew put it, "Fuzzy language leads to fuzzy thoughts": give a feeling a name and you can actually talk about it, instead of letting it curdle into something bigger.
The useful distinction is that a wibble is information, not a verdict. It tells you a nerve got touched — maybe you're tired, maybe you skipped a check-in, maybe you just want a hug and ten minutes. It doesn't mean the relationship is broken or that non-monogamy "isn't for you." The work isn't to never wibble; it's to notice it, say it, and let it pass.