We use this expression to say that we can’t explain or fully understand something.
You can use this phrase in ………… language.
A: She looks a bit different, doesn’t she?
B: Yes, I can’t put my finger on it. Has she changed her hair colour?
A: Why don’t you like Petra?
B: I don’t know. I can’t put my finger on it. I just feel she is a bit awkward.