an itchy trigger finger

1. An inclination or readiness to shoot a gun (especially at someone) without needing much or any provocation or justification. I wouldn't stay too long around here—there are a lot of people with itchy trigger fingers in this part of town. Well, you did leave Susie at the altar. Her father doesn't normally have an itchy trigger finger, but for you, I think he'd make an exception! I have no idea why he came back. He knows he's one of the most hated men in these parts—and that a lot of fellas here have an itchy trigger finger!

2. By extension, an inclination, readiness, or tendency to act or react carelessly, rashly, or without due consideration. John's itchy trigger finger is going to get him into trouble with his boss someday. When you're working in a restaurant, it's important not to have an itchy trigger finger when customers come to you with complaints. Look, you can't have an itchy trigger finger in a job like this—it demands calm and patience.