masc. proper name, from Latin Basilius, from Greek Basileios "kingly, royal," from basileus "king," especially the king of Persia, "prince," possibly from a language of Asia Minor (compare Lydian battos "king"), but according to Beekes, it "is no doubt of PreGreek origin (i.e., not a loanword from another country)." The youngest of the Greek words for "king" (alongside koiranos and anax). St. Basil the Great lived 4c. and was the founder of Eastern monasticism.

basil (n.)

aromatic shrubby plant, early 15c., from Old French basile (15c., Modern French basilic), from Medieval Latin basilicum, from Greek basilikon (phyton) "royal (plant)," from basileus "king" (see Basil). So called, probably, because it was believed to have been used in making royal perfumes. In Latin, confused with basiliscus (see basilisk) because it was supposed to be an antidote to the basilisk's venom.