Meaning of “Otsu(おつ / 乙)”

Basically, It means “Thank you for your hard work.” or “You did a great job.”.
“Otsu(おつ)” is an abbreviation for “Otsukaresama(お疲れ様)” and this is a slang.

Otsu(おつ) have two meanings.

One means “Thank you for your hard work”.
We use “Otsu” when we want to thank someone for his/her great work.
(This is a slang so it’s too frank to use against elderly people)

The other means “Thank you for your ‘stupid’ work”.
In this case, “Otsu” is a sarcasm.

Example Conversation with a friend

A: It’s time to go
B: Otsu

A: そろそろ落ちるわー
B: おつー

Example Conversation on the BBS (sarcasm version)

55[ID:000001234]:
　She’s so cute, check it this!
56[ID:000001234]:
　True!
57[ID:xxxhoge]:
　>>55 >>56 Same ID, sock puppet otsu!

55[ID:000001234]:
　この子めっちゃ可愛いから見てみて！
56[ID:000001234]:
　ほんとだ！
57[ID:xxxhoge]:
　>>55 >>56 IDいっしょじゃねーか、自演乙