His name in vain

That’s a good question. I don’t know myself, but I wonder if it might have evolved out of the invocation of the name of Jesus. Some people automatically invoke Jesus, Mary, or some saint when pained, shocked, or in despair. In those cases they aren’t using it as an explicative; It’s more of a cry for help.

Whether or not it’s seen as sacrilegious depends on the intent, I suppose. I can imagine people adapting the habit without meaning it as a plea at some point in the past, and some religious people seeing it as mocking their legitimate pleas, and taking offense. A lot of religious folks see this as a violation of the third commandment (or second, depending on your denomination) but that might be a misunderstanding.

The original meaning of the “Thou shalt not take the name of the Lord thy God in vain” commandment didn’t refer to this practice. At the time, it was meant as a warning not to use God’s name as a magic invocation, or at least not without good reason. Some ancients believed that, if you knew the name of a god, speaking that name aloud would give you some measure of control over it. Some scholars see this as the reason that the Abrahamic God never gives or is described by his actual name, but only by titles (although there is some thought that the Hebrew Elohim and El evolved out of one of the name of one of the Canaanite gods, El).