(What?! Did you forget we were speaking of old idioms?! I am truly saddened.)
Both of us had English-born grandfathers, and both of us lived with them for a time. Both grandfathers used the phrase.
In my grandfather's context, at least the way that I perceived it, the saying meant: "I have something to attend to, and it's none of your business." Or at least, "I have something to attend to, and I can't be bothered getting into an explanation of just what it is."
To Sue, it was a polite way of excusing oneself to go to the bathroom.
So, who got it right?
It turns out that we both did, for the Urban Dictionary lists both definitions. Of course, I humbly note that my version is listed first. This thrills me to me end as I am not used to coming in first. Thankfully however, I am mollified to know that "the last shall be first" on that somewhat postponed meeting in the sky.
A quick internet search also revealed that the same question was asked on the English Language & Usage site where I learned a few tidbits*. These are direct quotes from the answers provided. Some of these answers referred to other internet sources.
- The earliest confirmed publication is the 1866 Dion Boucicault play Flying Scud in which a character knowingly breezes past a difficult situation saying, "Excuse me Mr. Quail, I can't stop; I've got to see a man about a dog.
- The most common variation is to "see a man about a horse". Almost any noun can be substituted as a way of giving the hearer a hint about one's purpose in departing.
- During Prohibition in the United States, the phrase was most commonly used in relation to the consumption or purchase of alcoholic beverages.
- This has been a useful (and usefully vague) excuse for absenting oneself from company for about 150 years, though the real reason for slipping away has not always been the same. [...] From other references at the time [around 1866] there were three possibilities: 1) [the speaker] needed to visit the loo [...] 2) he was in urgent need of a restorative drink, presumed alcoholic; or 3) he had a similarly urgent need to visit his mistress.
Of these reasons [...] the second became the most common sense during the Prohibition period. Now that society’s conventions have shifted to the point where none of these reasons need cause much remark, the utility of the phrase is greatly diminished and it is most often used in a facetious sense, if at all.
Now . . . if you will excuse me, I have to go see a man . . .
*Tidbits is an interesting word, which prudish North Americans have alterred from the more British titbits. Both versions, refer to little bits of something, whether it be a little something to eat or a bit of news/gossip etc. Everywhere else they use the titbit variation. In Britain, many birds are called tits. This has sometimes led to some amusing (for puerile male minds anyway) exchanges on Flickr when Brits post bird photos.