what's the meaning of "a short measure"?

a short measure is a small amount...

Yes.

He was pouring his own, but in a pub, spirits are sold by law, by "measure".

This will vary from place to place, but if you're ever sold "short measure" in a pub, then you're not getting what you paid for.

When my bartender hears I'm about to go, he "pours me a short one." about a half a shot glass full. (Jamison Whiskey)
It gives me something to sip as he tallies up my tab, takes my money and brings my change.

He doesn't charge me for this extra.

Joe( His tip does seems to grow a wee bit as I'm getting off the stool.)Nation

PS: Yes, I know, you over there don't tip anybody. Let's not start that again.

Here's a tip for you. Follow the facts 17.30 BST Towcester. Put a monkey on it.

Au contraire, mon ami americain.

But we don't run up a tab. I think yours is a more civilised system, and they do the same in most European countries, but not this one.

We pay for each order, each round of drinks or whatever, and we often say, when ordering a round (if we are Scottish, say) "and one for yourself". This is an invitation to the barsteward to award himself a small tip....which might be repeated several times in the evening from one's party.

When I was younger, and that covers potentially many decades it has to be said, a pub measure for whisky (that's the way to spell it, America) was a quarter gill.
Later it became a fifth, then even a sixth in some places. But of course the price never came down.

I don't know what's in a "shot" glass, and I don't know what a gill is in cc either. And that's okay, as I don't drink the stuff any more.

Maybe that's why he gives you the "short one" for free. It's obvious he has taken more than a fancy to you, Joe. Do you get off your stool in a noticeably provocative fashion?

A normal shot glass is one ounce of spirit or liqueur. Although there are larger shot glasses around, the largest I've seen is a 4 ouncer.

In Ireland, I kept trying to give the bartender a tip, but they kept giving it back.. So I bought her a drink. She accepted that with no problem.