(one) ain't particular
One has no opinion or preference about something. ("Ain't" is a nonstandard contraction that is considered very informal.) A: "Where do you want to go for dinner?" B: "Wherever you want. I ain't particular."
ain't particular
1. Rur. doesn't or don't care. Tom: Would you rather have ice cream or cheese on your apple pie? Mary: Whatever you're having. I ain't particular.
2. Rur. doesn't or don't have a preference. Jane ain't particular. She'll use any old brand of soap.