Is "pinky swear" a childish expression? - No, but it is used mostly by children. Do adults use this expression? - Yes, jokingly. Adults don't do pinky swears they just swear as in 'I swear I will do such and such.' Context? Joking with friends, talking to children. Examples? The only example I could think of goes something like this. It's between you and your close friend, you're a piano player and you're going to play publicly soon and you want her to come and cheer you up. - (Your friend) I promise I will go to the concert no matter what. - (You) Pinky promise? You both laugh and maybe actually cross pinkies.

It is a childish expression. When you "pinky swear" you wrap your pinky finger around the other person's pinky finger and you both make a promise of some sort. It's a thing that children do. ("I promise to be your best friend forever and ever and ever.") You're never supposed to break a pinky swear. When you pinky swear, it means you really, really mean the thing you're saying, with all of the depth and intensity and seriousness that a 5 year-old has. (And 5 year-olds tend to be straightforward. They don't have conflicting priorities or multiple plans the way adults do.) Adults use the term in jest or as a metaphor to imply that they're as serious about the thing they're talking about now as they were when they were small children making promises. I wouldn't use it at a work setting, but in an informal setting people will use the phrase from time to time.

It can be, but saying Pinky Swear can mean you won’t break that promise. Depends on also who you’re talking to. You could say that to a spouse or family and friends, but I wouldn’t use it in a work environment or somewhere professional.

Adults do use the expression, but they normally use it ironically and not seriously. You would use it to make sure someone is telling the truth. For example Man 1- I swear I didn't eat your sandwich Man 2- Do you pinky swear? Man 1- Yeah man, I promise. Woman 1- I swear I will never go to the beach without you. Woman 2- Pinky swear? Woman 1- Yeah, that's our place.