[tflabo]

Ye shall not make any cuttings in your flesh for the dead, nor print any marks upon you: I am the LORD.
Leviticus 19:28

[steve86]

When one leaves Christianity one leaves a lot of graces behind.

[LouAvul]

The "cuttings" for the dead refers to grief stricken people mutilating themselves over the loss of a loved one. The "print any marks" is generally thought of to refer to superstitious people who got tattoos (or brands) to commemorate their false gods. (At least that's what the several commentators I refer to say.)

I don't see any Biblical prohibition against tattoos in general. I don't have a tattoo, and don't plan on getting one, but I don't think it's necessarily a sin. Do you?