Board Thread:Off Topic/@comment-74.101.236.56-20170321065847/@comment-26397825-20170511034401

Her back tattoo originally didn't have anything to do with the Grimm or the Maidens, but they retconned that when they introduced the Maiden storyline to the story. In the flashback episode of V3, which is set roughly 6 months before the events of the very first episode of V1, she doesn't have the tattoo until after she absorbs the Maiden powers and the Grimm beetle enters her body.

So it now does have something to do with the Grimm, if only to be a tattoo that tells other Grimm she's on their side.