Лингвофорум

Here I supply the knowledge from related etymological sources at first:

Rous.Etym.:

1) 'uncertain' ("...nepulné yascno...");

2) relation to PIE *ten- 'drag, pull' or *tes- 'chop, hew' doubted ("...tẽcne PSdEu *ten- 'tẽgti' abo PSl *tes- 'tesati, tẽti' naõrẽd");

3) Maybe related to Lith. tąsyti 'drag', iter. to tęsti 'id.' ("Ceiny tẽcne Lit. tąsyti 'tẽgti', iter. to tęsti 't.s.'")

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1) Vasmer's: 'unclear' ("...neyascno...");

2) Relation to PIE *ten- 'drag, pull' impossible;

3) Presumed relation to тесать (/t^jesat^j/)

4) Reference to тазать (/tazat^j/)
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W. Maxeck:

1) Presumably original PSL. *tasatei, whereas Rous. *-sc- intensive.

2) On tasiti III: related to Roussish tascati 'drag, pull'; probably rel. to Lith. tąsyti 'drag', iter. to tęsti 'id.'. The oddity of Lith. -ą-/-ę- discrepancy with Sl. -a- Maxeck acquits by its 'strangeness' ("...isolováno...", "...chybí k němu primární sloveso...")

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For the lack of reliable etymology in the referred sources I endeavour to develop a current hypothesis for which sake I bring the verb tiscati (and its aspectual forms tiscnouti, tiscti (sic!)) in, on grounds of attested meanings in Roussish of tascati as 'chase, hustle, chevy (a deer, cattle); hurl, cast' alongside with meanings 'drive, drive urge; hurry', and noun tasca 'scuffle; thrashing, drubbing', and the evidently concurrent meanings of tiscti (tiscati, tiscnouti) in its turn as 'pull, drag; press; squeeze, trample. hustle, drive hard' alongside with 'carry'. An approved relation to adjective noun téscn (PSl *toisk-n-) helps to allude the *-ei-, i.e. *e-grade in tiscati, i.e. PSl *teiskatei, and hence *o (as in *-oi-: PSl *toiskn-) might have been lengthened to *-a- in an investigated stem tasc- (PSl *tōsk-). Wintish dialect verbs тазать (/tazat^j/) 'scold, abuse' and тазить (/tazit^j/) 'search, grope, pick' may appear related hereto as well.