Originally Posted by
AdamFTF
In story, it was because it was the name of the other Thunderbolt backwards.
Okay, so in the Morrison JLA story, thunderbolt genies are 5th dimensional beings like Mr. Mxyzptlk. To summon them or send them away, the user has to say their name backwards. Johnny's original pink thunderbolt was named Yz, which would supposedly sound like "Cei-U" or "Say you" when arranged backwards and with vowels added to the mix. But in the story, there was an evil blue thunderbolt named Lkz (pronounced backwards sounds like "So Cool"). In order to stop Lkz, the JLA and JSA caused the two thunderbolts to combine into one purple thunderbolt. This thunderbolt inherited the Lkz name and magic word "So Cool".
Outside of the story, it's probably for the sake of a gag. One of the early gags in the Johnny Thunder strips was that he'd accidentally summon the thunderbolt when angry at someone by shouting "Say, You!" and then saying something he wished the offending person would do. Like "Say, you! I wished you'd go jump in a lake!" And then Thunderbolt would make them go jump in a lake. Since Jakeem is a kid and doesn't have those old timey speech patterns, they probably changed it to "So Cool" to make it more a gag based around awe and excitement. Like "That is so cool! I wish I had a bike like that!" Then Thunderbolt would "borrow" the bike from somewhere he shouldn't.