Casper Smart -- Blog Claims His Lawyer's a Dumb F***

Casper Smart's lawyer just got the most sarcastic letter we've ever seen in the 9 years of TMZ ... all stemming from Casper's alleged dalliances with transsexuals.

Casper's attorney threatened to sue TheDirty.com after 2 transsexuals posted comments on the website claiming they hooked up with Casper. The lawyer says the comments are defamatory.

But the lawyer for TheDirty -- David Gingras -- fired off a brutal response, telling Casper to pound sand. TheDirty's lawyer says if Casper has a case ... it's against the transsexuals, not the blog.

Here are some of the choice comments in the lawyer's response:

-- "In America, we have this annoying little thing called the 'First Amendment.'"

-- "First, your letter is addressed to 'Hooman Karamian' and yet it begins, 'Dear Mr. Hooman." I have no idea why lawyers think it's impressive to refer (incorrectly) to another person using their first name. I must have missed that class in law school. Do you think this makes you look smarter? Trust me -- not so much."

-- "Now let's get back to first year of law school, shall we? And to make this really easy for you …"

Casper's lawyer made the mistake of telling the lawyer for TheDirty this wasn't his first rodeo. So TheDirty's lawyer fired back, "You're darn tootin' Cowgirl -- this isn't our first time at the rodeo."

And check out the artwork next to the lawyer's John Hancock.

Them's fightin' words, cowgirl.