What does the expression to buttonhole someone mean?
A week before we were to be married, she asked me if I had purchased buttonholes for the ceremony...
Say what???, I thought to myself. "All my suits had their own buttonholes sewed into them...I see no reason to buy one for my suit."
Turning to my then fiance I said, "I'm supposed to get buttonholes???"
"Yes!!! and you had better talk to your mother to make sure they match her dress."
Exasperated I looked at her and said "What are you talking about my suit has buttonholes and they certainly match my suit???"
She disgustedly looked at me and said "I don't mean that kind of buttonhole. I mean the flowers you wear!!!!"
"Oh!!! you mean a corsage (for women) or boutonniere (for men).
A day later I was talking to my grandfather who was very English. "Did you know that Shelia wanted to make sure I talked to mother about the buttonholes I was getting for the wedding?"
He looked at me all knowingly and said "and well you should!!"
This started my education in British English
Those are two definitions of buttonholes but your question is different.
In this case as a verb, to buttonhole, means to grab someone (almost literally) for a close conversation about something.
John buttonholed me at the party to discuss our plans for the business meeting next day. Mary was not impressed.