Ed Sheeran -- X Marks the Spot ... Of the Worst Tattoo Ever! (PHOTO)

"Don't" is the name of one of Ed Sheeran's most popular songs ... which is what his friends should have told him when he said he was getting a giant lion tattoo on his chest.

Sheeran posted a photo of the new ink and captioned it, "Halfway and ouch."

The other half can't possibly be any worse ... right?