IPA: //ˌbuːtəˈnɪə//
KK: /buːtənˈjɛr/
A small flower or a bunch of flowers that is worn in the buttonhole of a jacket or suit.
He wore a red rose as a boutonniere for the wedding.
Boutonniere comes from the French word 'boutonnière', which is derived from 'bouton' (meaning button) and the suffix '-ière' (indicating a place or related to). The term originally referred to a buttonhole, specifically the place where a flower is worn, thus a boutonniere is a flower worn in a buttonhole.
Think of a 'button' ('bouton') where a flower is placed, and remember that a boutonniere is a flower worn in a buttonhole.