You can read about him in Isaiah chapter 8 v 1,2 and 3
God told Isaiah to make a big notice and write the word Mahershalalhashbaz on it in very clear writing so that everybody could see it.
It was a warning to all the people that their enemies, the Assyrians, were going to come quickly defeat them.
Isaiah and his wife then has a new baby boy and called him Mahershalalhashbaz
The meaning is “In making speed to the spoil, he hastens the prey”