"Kiasu" is a very common Hokkien word that is used in Malaysia. Literally, it means "fear of losing". It doesn't just mean being a sore loser. For example, it can be used to describe people who make sure they are the first to arrive at a food festival so they won't miss out on the 'good stuff'.
Hokkien is one of the many Chinese dialects (like Cantonese). You don't need to be speaking Hokkien to use "Kiasu". It can be slotted into any English sentence so I guess its a Manglish (Malaysian English) word as well. For example, Aaron is one very kiasu four year old!
In Aaron's case, its more of being a sore loser rather than being one of those Loser people who annoy other people. I can remember being a little kiasu when I was a child but he seems to find it much, much harder to lose than I can remember.
I noticed this behavior awhile back when I taught him how to play Tic Tac Toe. He tried his best to first, cheat, and then when I explained that was wrong, he tried to sweet talk me into making my moves according to where he pointed. Finally, he just refused to play. Since then I have consciously tried to play more games with him. The other day, when we played a game involving dice, he tried to place it with the 6 facing up whenever I turned around to take a look at Adrian! And today, with Snakes and Ladders, he conveniently side stepped all the snakes! Cheeky little guy!
I guess its a natural part of growing up and some people just feel the need to win a little more acutely than others. I think its good to be competitive but we also need to learn how to lose gracefully. Sometimes I know I find it hard to lose but I'm old enough to know how to hide it.