"The name Birdsfoot Trefoil for this plant is because the group of seed pods can resemble a birds foot. Although the flowers occur in groups of about 7, not all develop seed pods and there are frequently three slender pods joined at their base, resembling a bird's foot, with three long toes."
Dave's Garden
The trefoil part of the name refers to the leaves (foil - feuilles) which are each comprised of three (tre) leaflets at the tip and two below, on each leaf stalk. I guess they should actually be called cinquefoil, but maybe whoever named it didn't notice the other two leaflets???!!