It's not a tattoo, it's a birthmark.
4 seconds sound clip from the How to Train Your Dragon movie soundboard.
You can hear this line at 00:34:14 in the Blu-ray version of the movie.
Quote context
[...]
- All right, I'm off to bed. You should be, too.
- Tomorrow we get to the big boys, slowly but surely making our way up to the Monstrous Nightmare.
- But who'll win the honour of killing it?
- It's gonna be me. It's my destiny. See?
- Your mom let you get a tattoo?
- It's not a tattoo, it's a birthmark.
- OK, I've been stuck with you since birth, and that was never there before.
- Yes, it was. You've just never seen me from the left side till now.
- Hey, Toothless. I brought breakfast. I hope I hope you're hungry.
- We've got some salmon, some nice Icelandic cod and a whole smoked eel.
[...]