noun

• (archaic) That which is asked or granted as a benefit or favor; a gift or benefaction.
  "I gave you life. Can you not return the boon by giving me death, my lord?"
• (obsolete) A prayer; petition.
  "The wofull husbandman doth lowd complaine, / To ſee his whole yeares labor loſt ſo ſoone, / For which to God he made ſo many an idle boone."
• (British, dialectal) An unpaid service due by a tenant to his lord.
• (Hindu mythology) A blessing, typically a supernatural power, granted to an ascetic by a god or goddess.
  "A telling story is that of Vikra, who, after practicing severe tapas for many years, called on Śiva, asking him to grant the boon that whosoever's head he would touch, that person would die instantly."

adj

• (now only in boon companion) Gay; merry; jovial; convivial.
  "Greedily ſhe ingorg’d without restraint, / And knew not eating Death: Satiate at length, / And hight’nd as with Wine, jocond and boon, / Thus to herſelf ſhe pleaſingly began."
• (archaic) Kind; bountiful; benign.
  "With mazie error under pendant ſhades / Ran Nectar, viſiting each plant, and fed /Flours worthy of Paradiſe which not nice Art / In Beds and curious Knots, but Nature boon / Powrd forth profuſe on Hill and Dale and Plaine, / […]"
• (obsolete) Good; prosperous.
  "boon voyage"