It's Scottish for Alexander - the original spelling is with a D and some ****s will argue that all the other spellings are Anglicisations. Like:
I think Adil Rashid, Mudhsuden Singh Panesar and Ravinder Singh Bopara are Buckinghamshire names are they not?
It was in yer Murton guy's Clans of Scotland series - I'm pretty sure there's a mountain named Ben Alasdair or something which is the ancestral seat of some clan - McAllistair or the Alexanders or somesuch. While I'm on it, Neil Oliver's History of Scotland series has Episode 2 on the iPlayer the now about the 1715 and 1745 Jacobite rebellions - proper interesting and in a fairly modern context given the plums that are marching today behind a Scottish flag for a guy that saw Scots as at best, a nuisance and, at worst, a hindrance that he wouldn't have minded exterminating.
I don't have an issue with South Africans playing for England, they were nice enough to help us during the war after all.