Old Lithuanian plęšti.

Old Lithuanian plęšti ‘rejoice, jump in joy, dance’ may be interpreted as a back-formation from *plenšoti, a verb borrowed from Old Polish plęsać.

As is well-known, the oldest Slavicisms in Lithuanian are of Belorussian or Old Polish origin. Assuming that the OLith. sequence ęS < *enS is a faithful rendering of the Slavic nasal vowel ę before a fricative, it is easier to match Lith. plęš-/*plenš-with the 15 th -c. Old Polish form plęsać 1. 'clap, hit one's hands together', 2. 'jump, dance' (Słownik staropolski VI, 148), than to OBr. pljasati (var. plesati) 'dance, jump', where the root vowel does not have a nasal resonance. As regards the difference š : s, it is due to generalization in the Lithuanian paradigm of the consonant -š-characteristic of the Polish present form: pląszę, pląszesz, pląsze (cf. OBr. pljašu, OCS plęšǫ). The Lithuanian form is thus a kind of hybrid: its vocalism is that of the OPol. infinitive plęs-, Pol. pląs-, whereas its consonantism goes back to the Pol. present pląsz-.
There is, however, a certain detail that makes it difficult for a student of Lithuanian ultimately to accept the status of plęšti, pląšti as a borrowing. It is the primary form of the verb under study. 2 Specifically, verbs borrowed from Slavic are not usually disyllabic but trisyllabic, with the suffixes -yti or -oti corresponding to Sl. -iti, -ati, cf. e.g. slūžyti 'serve' (not *slūžti), pléntoti 'tangle, e.g. a thread' (not *plęsti). In Lithuanian transposition, OPol. plęsać, Pol. pląsać should have assumed the forms *plenšoti, *planšoti. That this is not the case might be due to the original transpositional forms *plenšoti, *planšoti having been modified in Lithuanian; more precisely, they would have undergone back-derivation to plęšti, pląšti. There does not seem to be an exact parallel for this alleged process in the class in -oti. On the other hand, one might point out two fairly certain quasiprimary formations in the class in -yti, cf. tróti ⇐ trõtyti 'lose' (from Br. trácicь) and tū́žti [įtūžti 'become angry'] ⇐ tūžyti 'worry, be sad' (from OBr. tužiti, Br. tužýcь). 3 Finally, one has to mention a synonymous word tañcius, tañčius 'dance', which is also a Slavicism in Old Lithuanian (i.a. in the texts of Daukša and Bretkūnas), and comes from OBr. tanecъ, -nca, Br. tánec, -nca (⇐ Pol. taniec, -ńca).
The current explanation updates the entry plęšti included in LED 989, a work -let me add -to which Professor Axel Holvoet selflessly devoted his precious time and creative effort as an editor.
Czech Go.
Old Belarusian OCS Old Church Slavic OLith.
Old Polish Pol.