This phrase is most often used to refer to the caber toss, a traditional Scottish Highland Games event in which a large, heavy log (the caber) is lifted, balanced, and thrown. The "feidh" part of the term likely refers to the shape of the caber, which resembles a deer's antler.
However, it's important to note that the term can also have a less literal meaning, being used as a metaphor for something long and strong, like a tree trunk or a mast.