A boomkin is a balance druid in the MMORPG World of Warcraft. The term comes from the talent whereing such druids are allowed to assume the form of a moonkin for the purpose of casting offensive spells.
A boomkin receives great spell crit and bonus damage and generates an aura which benefits all casters in the party. The term comes from oomkin, a once derogatory term used to reflect the fact that balance druids often went oom or "Out of Mana." This problem has since been resolved.
I wish we had a boomkin in this raid; my mage is gemmed for spell hit and I could use the crit aura.
N: A druid player in World of Warcraft that spams "MoonFire" constantly while in Moonkin form. The "Boom" comes from the sound MoonFire makes when it hits an enemy. Popularized by a youtube video.
"I went boomkin for the free flying mount."
"He just got owned by that NElf boomkin!"