ZZ Top's Billy Gibbons: David Letterman's Got Beard And Knows How To Use It

David Letterman just got the highest beard praise possible ... because it came from ZZ Top guitarist Billy Gibbons.

We caught Gibbons at LAX and asked him to rate Letterman's scruff on a scale from 1 to 10. The bearded legend, who's been rocking his own face piece since the late '70s ... gave us an honest assessment of Dave's.

Check it out ... Billy doesn't think Letterman quite has what it takes to pull off Ol' St. Nick just yet -- but he might wanna pick up a few guitar chords. Ya never know ...