Beaters are often discriminated and were often treated as unfair players that only care about themselves.
«Beater» is a portmanteau of the words «Beta Tester» and «Cheater», the "be" coming from "Beta Tester", and the "-ater" coming from "Cheater". Beta Testers have a bad reputation among the players of «SAO», since most of them had gone solo and did not bother to help the new players after the start of the cursed death game was announced.
The term was created after Kirito defined the difference between the «Newbies» and himself. He even refers to the other beta testers as newbies and not to compare them to himself, as he got to Floor 8, further than any other beta tester. This way, he chose to create this swear as a way of pushing the discrimination of SAO to the minimum. He ended up being the first person to be called «Beater», right after the Floor 1 boss fight.