Lately, for reasons that I may or may not explain in due course, I've been thinking a lot about the classification of words (parts of speech et al.). Normally what I do is consider the more nebulous categories (indefinite pronouns, quantifiers and alike), but today somehow I got to thinking about verbs and how - at least in Standard Average European and immediate surroundings - they tend to be easily identifiable based on morphological criteria only. And then I thought of "čau".
To explain: In addition to all the formal and semi-formal options, there are basically three standard informal greetings in Slovak: "ahoj", "čau" and "servus". The etymology is of course obvious and interesting in itself, the use of the first one tends to baffle German and English-speaking visitors to no end, but these are otherwise unremarkable interjections (Pauliny 1981:207). Well, not entirely unremarkable. You see, unlike with most other interjections in Slovak, when you use these to greet a group of people, you can add a suffix "-te", ending up with "ahojte", "servuste" and "čaute". What is that suffix, you ask? Why it's none other than the suffix of the second person plural imperative, i.e. a verbal suffix. This would fit nicely into the paradigm where second person singular imperative lacks overt marking for some verb classes (e.g. "rob" = do.2SG.IMP, "robte" = do.2PL.IMP), so one could argue that some Slovak interjections actually take some verbal suffixes.
Hang on, is that the only explanation? Well, no. One could for example consider the influence of analogy where these forms would be based on greetings like "maj(te) sa" which is a honest-to-Ninurta verb in the imperative: the full form is "maj(te) sa dobre" = "be well" "mať", lit. "to have", metaphorically "to be in a X condition". The absence of the reflexive pronoun might "sa" would have to be explained, but surprise surprise, "čaute sa" and "ahojte sa" do indeed frequently occur and from there, it's just one step to the forms we've seen, so that's plausible enough. We would thus have greetings formulas formed by analogy with an existing one and I don't think it matters that "maj(te) sa" is exclusively a farewell greeting, while the rest are universal.
But here's the thing: in addition to 2PL.IMP suffix "-te", there is also a 1PL.IMP suffix "-me" (Pauliny 1981:178). And guess what? Yes, you got it, "čau" and "ahoj" take "-me", too, to form "čaume" and "ahojme" and there is even one instance of "servusme" on teh intert00bz. These definitely cannot be explained by simple analogy with the "maj(te)" greeting, since appears to be no greeting formula "majme sa (dobre)". And so even if the 2PL imperative forms of "čau" and "ahoj" arose in analogy with "maj(te) sa" (which is possible, but in no way certain), they developed in their own way: first, they dropped the reflexive (which is not possible for "maj(te) sa" since the verb would then lose its idiomatic meaning) and once they could take one imperative suffix, nothing prevented them from taking the other one.
So, to sum up: some Slovak interjections take the full set of imperative suffixes. Now that is some awesome shit.
1. "Dobrý deň", lit. "good day".
2. "Zdravím", lit. "I greet".
3. See Swedish/Norwegian "ha det bra".
4. Note that in Slovak, one can inquire about other person's well-being by simply asking "Máš sa?". Normally this type of sentence would require a sentence-final adjective (for a simple question), a sentence-final interrogative particle (for focus on the particle) or a different word order (interrogative particle - reflexive - verb), but "Máš sa?" is perfectly cromulant, if informal, Slovak.
5. The main reason could be that the 1PL.IMP form of "mať" is not specific enough to facilitate a pragmatic interpretation as a greeting. In other words, when I hear "majme sa", I expect the next word to be the next part of one of the multiword expressions "mať" often features in, like "majme sa radi" ("let's like each other") or "majme sa na pozore" ("let's be careful"). Also, the imperative of "mať" is fishy in general.
Pauliny 1981: Pauliny, Eugen. Slovenská gramatika: Opis jazykového systému. Bratislava: SPN.