Should be careful with that, might confuse people, see also:

https://en.wikipedia.org/wiki/Confounding

which gets it wrong.

A variable with no detectable correlation with the outcome might still be a confounder, of course, you might have unfaithful things going on, or dependence might be non-linear. "Unlikely" usually implies "with respect to some model" you have in mind. How do you know that model is right? What if the true model is highly unfaithful for some reason? etc. etc.

edit: I don't mean to jump on you specifically, but it sort of is unfortunate that it somehow is a social norm to say wrong things in statistics "informally." To me, that's sort of like saying "don't worry, when I said 2+2=5, I was being informal."