According to docs filed by Shia, and obtained by TMZ ... his defense in the slander lawsuit is that name-calling might be "obnoxious and vulgar, but it's not defamatory." The whole thing started with a bizarre scene in April ... when Shia went off on the bartender in Jerry's Famous Deli.

Shia, who witnesses said seemed drunk, went off on the bartender for not serving him French fries -- which seemed like an act of racism to Shia. As he put it that night, "You f**ked up!!"

In his docs, Shia says the bartender's lawsuit -- which seeks $5 million -- should be dismissed because he was merely expressing his opinion, and his right to free speech allows him to do so.