The first slang term that I learned when in Jordan was telling someone “أنا عمك” which means I am your uncle. My host brothers used it all the time when joking around with each other and I didn’t get it at first, but I eventually caught along that it was just a way to roast someone. However, I brought it up in class to my teacher, and to my host parents multiple times and they had absolutely no clue why I would say that. So for younger people, this term is a good joke amongst your friends, but you would never say it to someone older.
The second slang word I remember using was “زلمتي” which loosely translats to my man (but it can be used for girls too (or at least my host brothers did)). This is something that we would just all use around the house to yell at each other, so we used it quite a bit. This is something that both my host parents and teacher understood when I asked them about it, but they said that they themselves wouldn’t use it. So once again this seems to be a pretty youth based word.
Generally Arabic is an interesting language because everyone speaks in what could technically be referred to as “slang.” There’s modern standard Arabic (MSA), which is what is primarily taught to non-native learners because every Arabic speaker usually has at least some base knowledge with (it it is called “فصحى” in Arabic which means “eloquent.” But in day to day conversation everyone uses عامية which is the regional dialect that varies depending on which part of the Arab world you’re in. People in the levant use different words than people in Egypt, who use different words than the people in the former French colonies, who use different words than the Gulf countries, etc. My host brothers even told me that when they’ve talked with people from Morocco or Algeria they often can’t understand them unless they both switch to MSA because the regional dialects are so different. This also adds an extra challenge to being an Arabic student because having a textbook knowledge of the language doesn’t do you a whole lot of good when they use entirely different words for things depending on what region you’re in. For example, when with my host family or if i was talking to someone on the street I’d use the words “ليش (laiish (why?))”, شو (shoo(what?)), and وين (weyn(where?)). However, in the classroom where we have to use MSA, you would get scolded for using those words instead of “لماذا (limaadha (why?))”, “ماذا/ما (maadha/maa (what?))”, and “أين(ayna (where?))”. So really everytime you have a normal conversation you’re technically using slang because the regional Arabic dialects differ so greatly, to the point that it sometimes becomes a problem for native speakers.