Sometimes verb in plain form can mean a command, often in the "manly way of speaking" used by Tenryuu among others. I think I've seen it followed by zo/ze most often, but this may be just my experience. Usually, it can be figured out the context, because here, translating it like "you/I (will) shut up" wouldn't make much sense. You can also use ta form as an imprative (and also "んじゃない/んじゃねー" as negative imperative, for example "するな" = "すんじゃねー"). Those things are tricky, because few English studying resources have them listed. Have this, (especially the first few pages) it's a good read.
