Is it farther or further?
early 15c., "impervious to water," from Old French estanche "firm, watertight," fem. of estanc "dried, exhausted, wearied, vanquished," from Vulgar Latin *stanticare, probably from Latin stans (genitive stantis), present participle of stare "to stand," from PIE root *sta- "to stand" (see stet). Sense of "strong, substantial" first recorded mid-15c.; of persons, "standing firm and true to one's principles" from 1620s.