An Etymological Dictionary of Astronomy and Astrophysics

1. (adj.) Describing a solution that is reduced in concentration.
   
2. (v.tr.) To make a solution thinner by the addition of water or the like.

Etymology (EN): From L. dilutus, p.p. of diluere “dissolve, wash away,” from → dis- “apart” + -luere, combining form of lavere “to wash;” cf. Pers. lur “flood” [Mo’in, Dehxodâ]
(variants Lori, Kordi: laf, lafow, lafaw, Tabari: lé); Gk. louein “to wash;”
Bret. laouer “trough;” PIE base *lou- “to wash.”

Etymology (PE): Owtâl, from Tabari utâl, “having water, impregnated with water, waterlogged,” from ow “water,” → water + tâl variant of dâr “having, possessor,” from dâštan “to have, to possess” (Mid.Pers. dâštan;
O.Pers./Av. root dar- “to hold, keep back, maitain, keep in mind;” Skt. dhr-, dharma- “law;”
Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE base *dher- “to hold, support”).