It is usually born out of frustration but he will just hit whoever is nearest, usually his big sister. He is 2.5 and she is 3.8. She is quite a gentle wee girl and they do get on great and play well together. But recently I've noticed he gets very possesive of his toys and hits any child who comes near whether they are trying to take the toy or not. Other times it's frustration e.g. if his train track isn't fitting together correctly, again he'll just hit out at his sister. Bless her rather than hit him back she comes running to tell us. We hold his hands and tell him that it's naughty to smack and if he keeps doing it then we remove him from the situation but it's getting that I can't go to anyplace where there are other children as he will stand his ground no matter how big the kids around him are. Sometimes I've even noticed him just giving his sister a gentle smack on the back for no reason whatsoever.
He's also taken to throwing things, well aimed just to bloody annoy us.
Are you all going to laugh at me and say welcome to the terrible twos? His sister was never like this.