What does the phrase "I have to talk to a man about a dog" mean exactly ?
it seems to me it's a polite way of saying I've got something to do but don't find it necessary to tell you about it (either because it's none of your busines or it's unimportant).
Could anyone confirm ?

 

Yes, you are correct. It's often used as a euphemism for "I have to go to the bathroom, I'll be right back."

 

Thank's Kat (warf !)

 

I've more often heard,"I have to talk to a man about a horse"
It does mean "I've got something to do but you don't need to know about it." I've always heard it in reference to needing to excuse oneself for a visit to the toilet.

 

Thank you everybody

 

It's not necessarily about going to the toilet. It can be about anything, a little talk about something you are not willing to disclose, so you are right with your first post, caroline R

 

Now that it has been confimed, what do we say in French for the same thing, I wonder? Anything similar (a cat perhaps)?

 

Ther is no expression you say when you go to the toilets ! But if somebody ask you 'where do you go' you can answer 'there nobody can go for me' (là où personne ne peut aller à ma place'. The joke is it's like you say it's very important. I have also heard this answer 'j'en parlerais à mon cheval' (i'll talk about that to my horse) to say that the subject somebody talk about absolutely doesn't interest you. If you want the person let you free, you can say 'vas voir là-bas si j'y suis !' (go there to see if i'm there !)