Have you ever wondered what the Z in for realz is actually doing there? I have. The dictionary form of the phrase is for real, without a final Z.
There is a (formally) similar expression, for keeps, in which the S is historically a plural marker. Apparently, the word keeps is short for keepies, and originates from some sort of game in which players collected marbles. The ones one won, one “kept”, and these became known as keepies. Possibly the phrase for keeps has contributed to the formation of for realz by way of analogy. Although admittedly, it sounds a bit far-fetched.
The Z in for realz seems clearly to be a plural marker (i.e. plural S), but why has it been added at all? There doesn’t seem to be any plurality involved in the semantics of the phrase, not even metaphorically. (It means ‘in earnest’, ‘really’, ‘truthfully’.)
We may get at a solution if we look at phrases like many thanks and many apologies, in which there clearly is a plural S on each respective noun. But here it makes sense. You can easily think of many acts of thanking or apologising, so here the plural meaning is semantically justified. The assumption is, of course, that the more you thank or apologise, the more sincere you are. However, when we say many thanks, we say just that. We don’t usually go on actually thanking multiple times (although that may happen, too). The point here being that the plural forms’ major function is to intensify or emphasize the act of thanking or apologising, not to mark a plurality of acts as such (which in these phrases would only count as a minor, secondary function).
It is conceivable that it is this intensifying function of the plural S that is being used in for realz. Hence in this phrase at least, English plural S seems to have developed a function devoid of plurality. If that really is so, then it would be interesting to see if this intensifying S pops up elsewhere in the language.
(Another explanation would be that there’s some cross-linguistic interference going on, namely, from Spanish de veras, in which veras is a plural noun with a Spanish plural marker S. If that’s the case, then it would seem that English has incorporated the Spanish plural S as an intensifier, and again, devoid of plurality.)