Tu as mis le doigt dessus !
tew ah meel DWAH dess-sew. Click below to hear this pronounced.
You’ve hit the nail on the head!
Only that’s not quite what it says! Instead, it’s an interesting inversion of another English expression: I can’t quite put my finger on it. But that one is negative, and this one is affirmative: You’ve put your finger on it.
I can’t explain to you why English speakers switch metaphors, from fingers to nails (but not fingernails), when we switch outcomes. The fact remains that when a French speaker wants to say that she can’t quite put her finger on something, she is more likely to say something like J’ai le vague sentiment que/de… (whether you use que or de depends on what follows–a clause or an infinitive). For example:
J’ai le vague sentiment qu’il cache un secret. I can’t quite put my finger on it, but I think he is hiding a secret.
J’ai le vague sentiment d’avoir oublié quelque chose. I can’t quite put my finger on it, but I think I’ve forgotten something.
As for dessus, it means on (it). It can be used alone at the end of a phrase, as it is here, the same way some English-speakers say Are you coming with? — ending their sentence with a preposition in brazen disregard for decades-old rules.
Where there are fingers and nails, there is likely to be a hammer. Luckily, there is no hammer in this equation, or it would likely be on mon doigt. Aïe !