Seth Rogen isn't backing down from his claims that Justin Bieber is "a piece of sh--," instead, he's doubling down on them.

During a Wednesday (March 12) appearance on Bravo's "Watch What Happens Live," Rogen was asked by host Andy Cohen about a tweet he posted following [article id="1720946"]Bieber's DUI arrest[/article] in January -- one that compared JB to a pile of excrement -- and, well, let's just say wasn't ready to apologize.

"In my opinion, Justin Bieber's a piece of sh--," Rogen said, as the audience cheered wildly. "He seems like he's obnoxious and ungrateful and insincere, and he puts people's lives in danger with his horrible, horrible behavior."

As you'd probably expect, those comments didn't sit too well with the Beliebers, who have been slamming Rogen all day with enraged (and, occasionally, grammatically correct) tweets of their own ... though we suspect Seth isn't sweating things. After all, he's got bigger things to deal with, namely promoting his new comedy "Neighbors" and attempting to [article id="1724061"]land a role[/article] in the upcoming "Star Wars: Episode VII."

Bieber, on the other hand, is currently [article id="1723928"]awaiting trial[/article] on a variety of charges, including DUI in Miami and [article id="1721446"]assault in Toronto[/article]. He also recently gave a [article id="1723801"]rather testy deposition[/article] in connection to a lawsuit filed by a paparazzo, who alleges Bieber and his bodyguard roughed him up last year.