Is it farther or further?
c.1300 (late 12c. as a surname, and Old English had purlamb "lamb without a blemish"), "unmixed," also "absolutely, entirely," from Old French pur "pure, simple, absolute, unalloyed," figuratively "simple, sheer, mere" (12c.), from Latin purus "clean, clear; unmixed; unadorned; chaste, undefiled," from PIE root *peue- "to purify, cleanse" (cf. Latin putus "clear, pure;" Sanskrit pavate "purifies, cleanses," putah "pure;" Middle Irish ur "fresh, new;" Old High German fowen "to sift").
Replaced Old English hlutor. Meaning "free from moral corruption" is first recorded mid-14c. In reference to bloodlines, attested from late 15c.
adj. pur·er, pur·est
Having a homogeneous or uniform composition; not mixed.
Free from adulterants or impurities.
Produced by self-fertilization or continual inbreeding; homozygous.