Pisher means one who urinates. But it is usually used in the phrase "a little pisher" which means either a) A child who is not toilet trained yet or b) A clever child; this can be used as an exclamation after a kid does something clever, and especially something sneakily clever. This isn't what you'd say when your kid brings home a great report card, it's got a tinge of sarcasm to it. A friend of mine had a son who, at age 6 or so, had cornered the market in his school on Pokemon cards - what a pisher!And then there's a joke to go with the word. It's not that funny a joke, so Madame L isn't going to add it here, but go to the site if you want to read the joke yourself.
Apparently the link here doesn't work for all the words, so if you're interested in any more wonderful and weird Yiddish words, try this site for "Yiddish Word of the Day."