When you call someone feisty, you mean they have a strong will and they will push back. They argue. They do not sit quietly if they think something is wrong. You say it with a smile: it is praise, or warm teasing, not a scolding. People often use it for someone small, young, or old who still has fire — a little dog that barks at a big one, a grandmother who tells the doctor what she thinks, a girl who will not let boys talk over her. Though actually, that last use can feel a bit uneven. Some women notice that men get called strong or tough, while women and girls get feisty, as if their fight is cute. The word is not cruel, but it can sound as if you are patting someone on the head. It is informal American English, from the late 1800s. You would not put it in a serious report.