To me, the purest purpose of an expletive is to convey that a feeling is so strong that it cannot be expressed in our normal words (whether this is an accurate assertion is a different argument).
Tonight, Baby W was complaining about something, possibly that she was too warm, so I took her out of her sleeping bag. As I hastily undid the shoulder snaps, I felt myself tugging a hair at the back of her head. I braced myself as, for a brief, silent moment, she absorbed what had just happened. Then she let loose a heartrending cry that expended all the air in her lungs and kept on going, soundlessly. She loaded up on air again for a forceful scream, followed by lots of swearing.
I’m pretty sure that babies swear.
I heard it when G was tiny, and I’m hearing it again with W. Normal crying is mostly vowels. Sometimes she says the classic “waaaah” when she’s pretty upset. But when she really gets mad, like she did tonight, she breaks out some more consonants, mostly “m,” “n” and “g,” and utters strings of miserable syllables. It tells a story of shock, misery, anger and betrayal, and it goes on for a while. Betrayal being a relatively new thing to her, she feels it keenly.