[memetherapy]

The consensus amongst the three English teachers I asked was that it should be "or else" or "otherwise" and that it's probably ungrammatical, although none of them could explain why. The one who is also an actor said "it works as a line" and she wouldn't think twice about it in a script or as written dialog. Interesting one though!

[dahart]

It's definitely not ungrammatical. But it is uncommon today, especially in General American English.