lilac

English lilac comes from obsolete French lilac (now lilas), borrowed from Arabic لِيلَك (līlak), which itself came from Classical Persian لِیلَک (līlak) — Persian nīl plus a diminutive suffix -ak.

The Persian root nīl traces back through Middle Persian nīl to Sanskrit नील॑ (nī́la) and ultimately to a Proto-Indo-European root, so a word that once pointed toward dark blue eventually settled into English as the name for a light purple flower.

The Persian and Sanskrit root behind lilac, nīl, is the same root that gave English the word 'indigo' its family of blue associations, though the facts here only confirm the shared ancestor, not indigo itself as an English cognate.