The story
Borrowed from Japanese 打(ぶ)っ掛(か)け (bukkake, “bukkake”, literally “act of splashing”), from the verb 打(ぶ)っ掛(か)ける (bukkakeru, “to splash (with a liquid) rudely”), from 打(ぶ)つ (butsu, “to hit”) + 掛(か)ける (kakeru, “to pour”).
Etymology from Wiktionary, CC BY-SA 4.0.