Justin Bieber to David Letterman: I Could Smell a Weasel

Justin Bieber taped a segment with David Letterman yesterday -- where the two teamed up to TRASH his alleged baby mama ... with Justin telling Dave, "I could smell a weasel, too."

Bieber proceeded to tell Letterman all about the process of supplying DNA for the paternity test -- saying, "Yeah, I took it ... They just swab your mouth."

Now, it's up to his alleged baby mama Mariah Yeater to supply her kid's DNA for comparison -- but Justin's not confident it will be a match ... telling Letterman, "I could smell a weasel."