Original: The name "buckwheat" or "beech wheat" comes from its triangular seeds, which resemble the much larger seeds of the beech nut from the beech tree, and the fact that it is used like wheat.
Simplified: Buckwheat seeds look like small beech tree seeds. This is how buckwheat got its name - buckwheat is a plant that is used like wheat and whose seeds look like "buck", or beech seeds.