Nicki Minaj ANNIHILATES Rapper -- 'Get Off My Metaphorical Penis!'

Nicki Minaj WENT BALLISTIC last night -- dropping some atomic hatred on a rapper who may have been talking smack about her ... depending on who you believe.

The drama involves a rapper named Ransom -- who famously worked with Nicki in the past on several songs.

Ransom recently released a new song called "Man Alone" which contained the lyric, "Before Nicki was wearing those crazy wigs, I was doing verses for her, just hoping she made it big."

After the song came out, there were reports on several sites -- including AllHipHop.com (a pretty reputable rap site) -- suggesting Ransom was basically saying he had ghostwritten some of Nicki's songs ... which is a HUGE insult. In fact, it's one of the worst things you could say to a rapper.

So, when we asked Nicki about the situation last night -- she went insane ... crushing Ransom as a liar who's "f**king desperate."

"Yo, I'm not even a man and n**gas got my d*ck in their mouth," Nicki said.

It goes on and she gets angrier -- you gotta watch the clip.

Now, here's the problem -- Ransom says the lyric was NOT meant as a diss. After the song came out, he tweeted, "As far as I know she writes her own [lyrics]."

So, is he suggesting that "doing verses for her" just meant that he was donating his time by rapping on songs for Nicki before she got famous? Or was Ransom saying he WAS her ghostwriter ... and now he's just backtracking??

Either way ... Nicki's pissed.