An Etymological Dictionary of Astronomy and Astrophysics

Moving or proceeding with little or less than usual speed or velocity.

Etymology (EN): O.E. slaw “inactive, sluggish;” cf. O.S. sleu “blunt, dull,” M.Du. slee, Du. sleeuw “sour, blunt,” O.H.G. sleo “blunt, dull,” O.N. sljor, Dan. sløv, Swed. slö “blunt, dull.”

Etymology (PE): Âhesté “slow, quiet, tender, soft,” ultimately from Proto-Iranian *ā-hasta-ka-, literally “at rest, motionless, seated.”
The first and third components are affixes, the main component from *had- “to “sit, be seated;” cf. Av. had- “to sit” (nī…hazdiiāt “would sit down”);
Pers. nešastan “to sit;” PIE base *sed- “to sit;” cf. Skt. sad- to sit,” sidati “sits;”
Gk. hezomai “to sit,” hedra “seat, chair;” L. sedere “to sit;” O.Ir. suide “seat, sitting;” Welsh sedd “seat;” Lith. sedmi “to sit;” Rus. sad “garden;” Goth. sitan, Ger. sitzen; E. sit.