not a word, but what casual slang (that's not offensive) do you know in Italian?
slang in italy depends very much on where you live, because we have tons of different regional languages and they are completely different from each other, but some expressions and sayings that we all use are:
che palle! = means “what a pain!” used when something mildly bad happens, it can be considered a swear word but it’s used by basically everyone over the age of 10
che figata!/che figo! = means “that’s cool!”, some parents consider it a swear word as well but it’s rare
basta! = means “that’s enough!”, not really slang but one of the most useful italian expressions
boh = means “i don’t know & i don’t care”, again, this is extremely useful and everyone uses it all the time
fré/frà = literally “bro”, used just as you would use bro
bella fré/frà/raga/name of someone! = just a way to say “hey”, used especially among young people
raga = means “guys”, used as you would use guys
presa male/presa bene = used when you’re sad/happy about something that just happened, used by young people
scialla! = means “take it easy!”, used by young people
cazzeggiare = means “chilling”/”messing around”, can be considered a swear word
beccarsi = literally “to peck”, means “to see someone around”, used a lot
figurati! = means “no need to worry about it!”, used when you do someone a favour and they thank you
magari! = means “if only”/”i wish”
mi fa cagare! = kind of a swear word, means “it sucks!”, literally “it makes me want to take a shit”
che schifo! = just like above, a bit less rude
dai! = meaning “come on!”
meno male! = means “thank god!” but not literally ( thank god = grazie a Dio)
essere un po’ fuori = means “to be a little crazy”, not really offensive, literally “to be a little out (of your mind)”
ci sei o ci fai? = means “are you crazy or what?”
mettersi insieme = means “to date someone”
mollare qualcuno = means “to dump someone” as in breaking up
cade a fagiolo! = basically a metaphor for precision, literally “to the bean”
avere un chiodo fisso = means “being fixated on something”, literally “having a nail fixed (on your head)”
come dio comanda = means “as things should be done”, literally “as god commands”
non t’allargare! = means “don’t go too far”, used when someone is pushing it with their behaviour a bit too much
parla come mangi! = means “keep it simple”, literally “talk as you eat”
chissenefrega/chi se ne frega = means “who cares”, some people don’t like it because it comes from the fascist era
if any italian wants to add something, feel free to do it!