The word comes from the French “boutonnière”, from bouton which means “button”. The British English term for this, “buttonhole,” shows a literal translation of the original French. French boutonnière buttonhole, from Middle French, from bouton button + -ière suffix for things that facilitate the use of that denoted by the stem.
A boutonniere is a flower or floral decoration which was traditionally pushed through the buttonhole of a jacket, but in modern times is most often pinned onto a gentleman’s formal suit coat or tuxedo jacket lapel.
While worn frequently in the past, boutonnieres are now usually reserved for special occasions for which a dinner suit or formal attire is standard, such as at proms, homecomings, funerals, and weddings. (Women who wear jackets on these occasions also often may wear boutonniere, but more typically a woman would wear a corsage.)
It is an American prom custom for the female to purchase the boutonniere for her male date and for it to match her dress and corsage. At American weddings the ordering of the boutonniere for the groom and wedding party is done by the wedding florist. The groom, groomsmen and other male participants in wedding party will almost always wear a boutonniere on his left side of formal suit coat or tuxedo jacket lapel.
When florists design a boutonniere with leaves and other treatments, they will often use a lot of wire, so you want the pin to be right near the base of the boutonniere head to avoid the wire. Pinning the boutonniere from behind the coat lapel and through the stem of the boutonniere so pin is not visible.
It is a British practice to place, after a wedding, the flowers from a buttonhole (together perhaps with other wedding flowers), on the grave of a recently deceased member of one of the families concerned.
It is a Polish wedding practice that the groom usually wears a fitted suit with a bow tie and a boutonniere that matches the bride’s bouquet.
Today,on less formal occasions, you can still use the coat lapel buttonhole without pinning your boutonniere, provided the lapel buttonhole is cut.