Another word for a posy is a nosegay.
This seems like an impossibly made up word. Like oventit (a bird often found in crosswords). It was made to fill up crossword clues.
Not a 1450s-ish English contraction of nose (meaning nose) and gay (meaning bringing joy), to make something which brings your nose joy which is clearly preposterous; not a word commonly used in bridal shops… but a sodding mountweazel if ever I saw one.