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1.
"Hi" in Japanese, informal
it's okay to use ossu with friends.. but not really with people of high status =P
by Farfello April 28, 2004
 
2.
1) informal greeting way among young male friends. Should not be used to greet older people.
2) Some Karate practicing people may be using this term in place of such as "yes sir*. However, it seems this word is a recent invention and not so much used among degnified old Karate-ka.
1) When two boys meet, they may say "ossu" "ossu" like "Hi" "Hello".
2) Karate-ka 1 show example, another may say "ossu".
by Ramunenakayam October 27, 2007
 
3.
"ossu" is a term used on the irc channel #GAYM on the ChatSpike network to replace osu (or osu!), which is a rhythm game where you move and click your mouse to the beat.

The reason this term is used is because the bot in #GAYM will remove your ability to talk in the channel if you say "osu."
<Guy1> Let's play some osu!!
*** GAYM sets channel #GAYM mode -v Guy1
<Guy2> ossu sCK
by Detrimentalist August 07, 2011