I'm not sure if this is the context where you met this phrase, but this is one interpretation: Let's say you and your friends are taking an important exam today. You've worked hard, you've said a few prayers, but you would still like a little bit of extra luck. So what do you do? Some people might put on a 'good luck charm' like a special ring or necklace to bring them luck. Others might put on a certain garment that they're sure will help them get a good mark in their exam. Maybe their favourite pair of socks? Or maybe lucky pants. Pants in British English is underwear. If you put on your special 'lucky pants' under your clothes, nobody can see them. But you know you're wearing them, and they might just help you pass that exam! Sportspeople are also famous for this kind of behaviour. Some sportspeople have special little rituals that they do before matches, races and competitions, and others have clothing - like pants or socks - to bring them luck.

Just an expression which means the same as saying "lucky." Not a common expression.

thank you Samuel and Su.Ki. I think this phrase has different meaning in American and British.anyway I got that .thank u again.