There are no Celtic symbols that mean "brotherhood;" however, the Brother of the Arrow is generally substituted as one. The Brother of the Arrow is a Celtic symbol that was used on warriors who were heading out to war, and means masculinity, power and bonds.
Like most Celtic symbols, there are many variations of the Brother of the Arrow. The most common version includes a rounded arrowhead that looks similar to a spade. The bottom of the arrowhead is generally comprised of swirls or loops that may or may not flow through each other. Brother of the Arrow symbols are generally used as tattoo designs.