(I) can't rightly say
I don't know with certainty. A: "How many people are coming to dinner tonight?" B: "I can't rightly say—my family tends to just show up without responding to invitations."
(I) can't rightly say.
Rur. I do not know with any certainty. Fred: When do you think we'll get there? Bill: Can't rightly say. Bob: Okay, how does this look to you? Bill: I can't rightly say. I've never seen anything like it before.