In The Last Jedi, I thought it was a little odd that Luke referred to the lightsaber as a "laser sword". Given that the lightsaber identifies a Jedi, why did he refer to it as such?
He was mocking Rey.
He was arguing to Rey that he wasn't the savior she thought he was, and he wasn't going to return and go defeat the First Order on his own. He mis-identifies the lighstaber to mock the idea that he could take down the whole army with his weapon. Laser sword is the way somebody who is unfamiliar to a lightsaber might refer to one, so it emphasizes the lack of knowledge someone would need to have to expect him to defeat the First Order.
He tries to remove the lightsaber's mythical image.
The term lightsaber implies a lot of things beyond the actual technical device. For example, it is a part of the Jedi religion, it is a weapon of mythical reputation, it is the weapon that great heroes fought their heroic battles with.
Luke wants to make Rey understand that everything beyond the apparatus itself is pure imagination, so he calls the lightsaber by a name that describes the device, but lacks context, to relativize her associations regarding the weapon.
It's a bit like calling, say, Excalibur, an “old sword”. It's aimed at removing properties of the device which are – in Luke's opinion – not real, and thereby questioning the validity of the Jedi religion that he has become disillusioned with.
Remember that we open with Rey handing Luke the lightsaber (i.e. the end of The Force Awakens) and
Luke tosses it behind him and walks away without saying a word, much to Rey's astonishment
Luke doesn't want anything to do with Rey, or rebellions, or Jedi. He's now a crotchety (and snarky) old man. Mocking Rey by disparaging the lightsaber is a way to discourage her so she'll leave him alone.