A saying to describe an item of clothing someone is wearing which is clearly too small for him. This is on the knowledge that if your are wearing something that is your wee brother's (little brother's) then it will be too small as your little brother would be often a lot smaller than you. This term is also used less commonly for something too big for you in which you would call your "Dad's top" or your "big brother's" top, again going on the knowledge that these people are probably a lot bigger. Obviously the term "top" refers only to a Tshirt but it can be used similarly when describing trousers as your "wee brother's trousers" but this phrase is not often used. To use this phrase on somebody is seen as an insult insinuating that either they cannot afford their own clothes or that they are too fat to fit into a certain garment of clothing. It usually results in laughter for those observing, as well as the victim of the joke being embarrassed and hard pressed to think of a come back.
That's rich coming from the guy wearing his Wee Brother's Top!
Fucking hell, are your Wee Brother's Trousers tight enough at the bulge
God sake man, what size is that, it looks like youe Dad's top