ulterior motive

a secret purpose or reason for doing something
不可告人的動機／原因／目的（等等）

• He claims he just wants to help Lisa, but I suspect he has an ulterior motive.
  他聲稱只是想幫助莉薩，但我懷疑他有不可告人的動機。
• She must have some ulterior motive for being nice to me – what does she really want?
  她對我這麼好，一定別有用心。她到底想幹什麼呢？

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PS I’ve always wanted to share with my students some useful words and expressions that help express ideas more easily, and this chance finally came! If you’re looking to expand your vocabulary, be sure to have a vocab notebook – it’s simply the best way to do so. I’ve kept a vocab notebook for more than 2 decades. Imagine that! When I was a student, every time I came across a good word, I would add it to my notebook. This old notebook is not only a record of all the words and expressions I’ve learnt but also something very precious I can’t afford losing, as I have benefitted so much from it. I hope you find your vocab notebook useful too!