Howard Stern fans are gonna LOVE this clip -- Jake Gyllenhaal, on the other hand, might not ... because his former co-star Beetlejuice is talking some serious smack.

We ran into Beet in NYC yesterday and asked about his Shakespearean performance in the 2001 movie "Bubble Boy" -- and it's pretty clear he's not a huge fan of Jake.

BJ said, "We got to know each other. When he sucked my c**k."

He added, "[Jake] got knocked the f**k out 'cause I hit him!"

Now, 'Stern Show' listeners know it's hard to take anything Beetlejuice says with more than a grain of salt -- he's known for totally making up stuff. But at least the guy is entertaining as hell.

If like us, you doubt the 4'7" Stern Wack Packer could really knock out 6-foot Jake -- you gotta hear what Beet says about pumping iron.

You're welcome, Stern fans.