Japanese slang isn’t all that different from what we use in English. We tend to do things like shorten the word, combine two words together (portmanteau) or use a word that has a similar meaning or association. Same thing in Japanese. However, understanding the meaning might be harder if you haven’t grown up with the language.
I’m gonna start today with a few casual greetings you can use with friends. Like with all Japanese, context is very important. Saying baka (ばか) for example in a fun tone to a friend can just equate to a joking ‘dumbass’, ‘idiot’ or ‘fool’, whereas saying it angrily to a stranger could mean something harsher like ‘dipshit’ or ‘fuckwit’. Context is key. So be careful when you use some of the slang, since in some situations it can mean an entirely different thing. Take note that a lot of Japanese idioms and expressions can be confusing or too simple when translated literally, usually there’s some meaning or connotation behind it.
So, useful greetings, just a few to start, I’ll do some more later.
OSSU (おっす) – Hey / Wassup? / Yo
おっす is an old military word that was used in the same way as ‘SIR, YES SIR’. A generous helping of ‘OSSU!!’ was given when a recruit’s name was called or was asked upon. With Japan changing and it’s military persona fading slightly, the word has regressed a little and has morphed into a new meaning.
Now, it’s used a lot by the youth of Japan. It’s meaning is a little vague, but generally it’s like saying Hey or Hi to a friend as a greeting. A general reply to this is a variation of the word, ussu (うっす) which is similar in meaning, probably closer to ’sup.
ZAIMASU (ざいます) – Mornin’
This isn’t a word per sey, mearly a shortening of a normal word. Ohayou Gozaimasu is Good Morning in Japanese (as many of you know). ‘Ohayou Go-’ has been chopped off (much like the ‘good…-g’ in Good Morning) and you’re left with zaimasu. You’ve got to pronounce this with a slur at the beginning, which would probably suit it considering it’s a lazy way to greet someone in the morning when you’re probably half asleep and can’t remember much Japanese! This word, again, is casual, although its used all the way up to a work environment. Teachers in schools often use it with one another at a morning meeting or when passing in the hallway on the way to first lesson.
SUMAN (すまん) – ’scuse me!’ / pardon
すまん is slightly different to sumimasen (すみません) as it’s only really used in the negative sorry! excuse me! form. I’m not sure if you could stretch so far as to use it as a thanks! cheers! meaning or not. Still, it’s another shortened word. For an ever so slightly polite version, lengthen suman to suimasen. I usually use that when I bump into someone or saying thanks to a shopkeeper/till assistant at a コンビ二 or something.
JA NE (じゃね) / MATA NE (またね)- See ya! / Later / etc.
I suppose these aren’t really slang, but they’re the normal casual way to say bye! Rather than Sayounara (さようなら) which sounds a little prissy or formal, these are used among friends you’re likely to see soon. Ja ne translates to Well then… and mata ne is once more? / again? / later?.

おっす is actually a contraction of おはようございます ^^
Never heard ざいます being used before. Did they use it a lot in Kansai?
すまん is a cool way of saying sorry casually. I use it quite a lot I guess.
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Yeah, ざいます was used a lot, although probably only out of extreme laziness, haha.
Yeah, it originated from おはようございます, there’s a little more about the origins of ‘おっす’ here if you’re interested. It’s in Japanese though.
Still not sure about すまん being used as ‘thanks’ or not?
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I think すまん could be used as ‘thanks’ the same way that すみません could be ^^
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Ja ne was the only one I knew, but I know ‘em all now.
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