Meaning: to have a lisp
Literal Translation: to have a hair on the tongue
Here’s a couple of examples of this expression being used in context:
Mes amis se moquent de moi parce que j’ai un cheveu sur la langue.
My friends make fun of me because I have a lisp.
Comme elle a un petit cheveu sur la langue, elle a quelquefois du mal à se faire comprendre.
Since she has a bit of a lisp, she sometimes has difficulty making herself understood.
Although this idiom is slightly informal, it isn’t cruel or offensive.
As well as this expression, you might also come across the informal (but not offensive) words le zozotement (“lisp”) and zozoter (“to lisp”, “to speak with a lisp”).
If you do want to talk about a lisp without this informality though, you can use zézayer (“to lisp”, “to speak with a lisp”) or le zézaiement (“lisp”).
Avoir un cheveu sur la langue is just one of a practically endless number of French idioms. If you’re interested in learning more, then check out my recently published Kindle e-book The Little Book of French Idioms, which introduces you to well over 100 common idioms. Each of these expressions is given with its meaning, literal translation, and – unlike a lot of other similar books – an example of how to use it in context.