Is it ensure, insure, or assure?
buttonhole but·ton·hole (bŭt'n-hōl')
A short straight surgical cut made through the wall of a cavity or canal.
The contraction of an orifice down to a narrow slit, as in mitral stenosis.
To get someone's attention as if by taking hold by a buttonhole: listening to and buttonholing other researchers
[1880+; Button in the same sense is attested from the early 1860s]