Etymology 1 
From Middle English boon (“prayer”), from Old Norse bόn (“prayer, petition”), from Proto-Germanic *bōniz (“supplication”), from Proto-Indo-European *bʰāni-, *bʰā- (“to say”). Influenced by boon (“good, favorable”, a). Cognate with Swedish bön (“prayer, petition, request”), Danish bøn (“prayer”), Old English bēn (“prayer, request, favor, compulsory service”). More at ben.
boon (plural boons)
- (obsolete) A prayer; petition.
- (archaic) That which is asked or granted as a benefit or favor; a gift; a favour; benefaction; a grant; a present.
- A good; a blessing or benefit; a great privilege; a thing to be thankful for.
- Finding the dry cave was a boon to the weary travellers.
- Anaesthetics are a great boon to modern surgery.
- (UK dialectal) An unpaid service due by a tenant to his lord.
Etymology 2 
boon (not comparable)
- (obsolete) good; prosperous; as, "boon voyage"
- kind; bountiful; benign
- gay; merry; jovial; convivial
- a boon companion
- Which ... Nature boon Poured forth profuse on hill, and dale, and plain — John Milton
- A boon companion, loving his bottle — John Arbuthnot
Etymology 3 
From Gaelic and Irish via Scots.
- The woody portion of flax, separated from the fiber as refuse matter by retting, braking, and scutching.
Part or all of this entry has been imported from the 1913 edition of Webster’s Dictionary, which is now free of copyright and hence in the public domain. The imported definitions may be significantly out of date, and any more recent senses may be completely missing.