And what is the meaning of the plural??

Well, you know, the word itself is one from a philological grammatical tradition. Tmesis. in Greek, originally meant 'cutting'. It's related to the words atom (indivisible, uncuttable), -tomy (as in appendectomy), etc. What I take the plural to mean is more than one example of the morphological process of tmesis. (BTW, in Greek, Sanskrit, and Germanic, some languages which have tmesis, separable verbal prefixes can detach under specific conditions from the verb. These verbal particle looks suspiciously like prepositions, but act otherwise. I learned the grammatical meaning of tmesis decades before it was given its expanded meaning of sticking expletives in-between the syllables of exclamations. Regardless, the established plural of tmesis is in established.

Ceci n'est pas un seing.