The Symbolic Knife
A knife tattoo can express a feeling of sacrifice. This conveyance is ancient, and deeply rooted in many religious/spiritual sects in which sacrifices have been (and still are in some cases) practiced for eons.
In Buddhist symbology the knife often represents cutting the ties of materialism.
In the suit of swords in the Tarot. This suit represents challenges, usually wrought from mental nuances like doubt, fear, worry. We see a constant struggle to slice through confusion. The knife in these cards symbolizes the double-edged nature of the mind.