“I got it on a whim one day a few years ago.”
You have a tattoo on your arm. You didn't think very carefully about it when you got it. You're on a date, and your date asks about your tattoo.
I got it on a whim one day a few years ago.
"A few" is a number that's not specific, but it usually means somewhere between 3-10. It's a little less formal than "several" and also sounds like a slightly lower number. Here are some examples:
A "tattoo" is a drawing that's permanently drawn on your skin with a needle.
Use the word "get" with "tattoo". "Getting a tattoo" means that you pay someone (or ask a friend) to tattoo you.
When you have a small, temporary feeling of wanting to do something, it's called a "whim".
Doing something "on a whim" means doing it wihout planning it carefully or thinking about it seriously. You do it just for fun.
"On a whim" is a neutral phrase: not positive or negative, and neither very formal nor very casual.