Get off!

Hi! Please, is it correct say "Get off!" or "Get off it!" ? 

There are many meanings depending on the situation when used as slang.  "Get off" and "Get it off" by looking at the sentence only without context, means to get something off you.  The first one is direct and the 2nd one is passive.  The 2nd one has a slang meaning to mean "self abuse" or "self pleasure as goaded by another person!  "Get off it!" in means "to stop doing it".

See also

http://www.urbandictionary.com/define.php?term=get%20off%20it

http://www.urbandictionary.com/define.php?term=get+it+off

http://www.urbandictionary.com/define.php?term=get+off

Get off!

Hi! Please, is it correct say Get off! or Get off it! ?

Just wondering if you meant to say "Come off it!" 

You really need to give the context!