A couple of folks have asked about the meanings of the tattoo on my left leg. Here is some insight into Celtic shield on the middle of my calf… My wife and daughter often speak of me as the family protector, which brings special meaning to this symbol in my tattoo sleeve.
The shield Celtic knot is a symbol for protection. The ancient Celts used it to decorate the shields of warriors, the clothing of children, and to protect the sick.
Paul Michael Raspa, Jr. 
I have had the same tattoo on my left shoulder/upper arm for about seven years. Got it before Iraq and I’m still kickin’.
The Celts themselves left very little in the way of records, and most symbols are interpreted by archaeologists and other scholars who study the symbols in context. Some ancient Celtic symbols have changed in meaning over time, having been influenced by the introduction of Christianity and the influence of other cultures. A general rule of thumb is: the shape of the design often determines the “meaning” of a knotwork design– triskele and trefoil shapes should be regarded as triangles and trefoils the threefold dominions of earth, sea, and sky. Squares or four-fold shapes are shield knots, symbols of protection from spirits or malevolent influence.