Albert Barnes Commentary Mark 3:17

"and James the [son] of Zebedee, and John the brother of James; and them he surnamed Boanerges, which is, Sons of thunder:" — Mark 3:17 (ASV)

Boanerges. This word is made up of two Hebrew words, signifying sons of thunder; meaning that they, in some respects, resembled thunder. See Barnes on Matthew 1:1.

It is not known why this name was given to James and John. They are nowhere else called by it. Some suppose it was because they wished to call down fire from heaven and consume a certain village of the Samaritans (Luke 9:54). It is, however, more probable that it was because of something fervid, and glowing, and powerful in their genius and eloquence.