What does this tattoo character mean?
The character is 安 (safe; secure; content).  By itself, it is innocent enough.

However, "安" is also the last character in the name of a notorious Hong Kong Triad gang "新義安" 

In short, this  tattoo reminds me of this particular crime syndicate, consider it is on a bicep-- a body part that represents power and strength...

If the owner of this tattoo is a Hong Kong Chinese, I would stay clear of him, just in case.