Billie Eilish Shows Off Full Back Tattoo to Mixed Reactions

Billie Eilish is showing off the full version of her massive back tattoo, a piece she's teased before ... and fans are pretty mixed on their thoughts.

The singer posted a pic Wednesday as part of a larger October photo dump -- and in it, she's laying on her stomach at what appears to be a tattoo shop. Billie's got her bare back exposed, showing some sort of intricate design that runs down her spine -- and it looks fresh.

Unclear if this is a shot that she's had in her camera roll for a while ... we've seen glimpses of this tat before, but have never seen the entire thing. This is either Billie showing that she's gotten more work on it recently, or just a flashback to when she first got it done.

In any case ... the tattoo here looks a little smeared and incomplete. There've been mixed reactions to the ink online -- some dig it, others aren't big fans.

FWIW, this is just one of a handful of tats she has ... Billie has one on her chest that reads "Eilish," another one of a dragon on her thigh and yet another showing fairies on her hand.