Spartan Laser typically refers to the weapons ungodly weight and destructive capability. As an anti-armor weapon, the laser needs a powerful source of energy. And since the UNSC hasn't been able to shrink the fusion reactors in the MJOLNIR armor sets from American football size to say, that of your palm, the next best thing is a Lithium ion battery. And those f*****s are HEAVY. And since the battery has to power anything between four to five shots (power may be adjustable: see "The Package" in Halo Legends), that battery is going to be heavy as hell. Hence, the name Spartan Laser.