ulterior
adjective uk/ʌlˈtɪə.ri.ər/ us/ʌlˈtɪr.i.ɚ/
ulterior motive, reason, purpose, etc. 不可告人的动机／原因／目的（等等）
a secret purpose or reason for doing something
He claims he just wants to help Lisa but I suspect he has an ulterior motive. 他声称只是想帮助莉萨，但我怀疑他有不可告人的动机。