It's a shame that he never uses that term anymore - it's kind of a trademark of his, but one that he's let lapse. Personally, I think it would be kind of cool if 'chum' was basically the word he uses for either a friend or an enemy, but nothing in between - for example, if he uses it while palling around with the Batclan, it's kind of endearing; if, on the other hand, it's something he says to a criminal from between gritted teeth in a dark alleyway, it is ABSOLUTELY TERRIFYING. In other words, you'll know just where you stand with Bats by how (or if) he uses his special word.