It is a little different than "be furious or outraged". You are angry when you "snap" at someone, but snapping can have two different meanings. The first meaning is when someone "snaps". This means that someone was having a hard day or a lot of things were building up and now they reached the point where the person couldn't take it any more. When someone suddenly blows up and gets angry at others, they "snap". "I have had such a bad day. I swear, I'm going to snap." The second meaning is more of "snap at someone". When someone does this, it is because they are irritated or frustrated, but not necessarily at the speaker. When someone who is angry/irritated says something short, mean, angry, etc at someone else, they are snapping at them. "My friend was irritated earlier and snapped at me for asking her a question." I hope I was able to help. If you are still confused, please let me know and I will try to explain it better ^^

We use "snap" to describe how a crocodile bites. Maybe that will give you right image when someone "snaps at you". :)

You know how a snap is a quick, sharp, loud, forceful, burst of sound? When you speak in the same tone - quick, sharp, loud, forcefully - you're snapping at someone. You usually do it when you're angry or frustrated or upset. You can usually only snap a word or two. Anything longer and you're yelling, not snapping.