Neat is a word which, in one of its more archaic uses,
refers to cattle. Neat's-foot oil, sometimes referred to as
neatsfoot oil by people who can't be bothered with punctuation, is
made by boiling the foot and shin bones of cattle (minus the
hooves) in water. The oil, which is yellow and fatty and comes from the marrow of the bones, is skimmed from the surface of the water as the mixture cools.
Neat's-foot oil is widely regarded as one of the best oils for the
preservation of leather. Because it is relatively difficult and
expensive to obtain pure neat's-foot oil, one usually finds it mixed
with other oils. It is used in saddle soap, and is a principal
ingredient in many leather polishes. It is often applied, mixed with
lanolin, to leather bookbindings to keep them supple. Sometimes people use it to break in baseball gloves. It is also widely used in the process of tanning hides.
You can find neat's-foot oil at hardware stores and saddlery shops. It should be used sparingly, as too much of it can make leather soggy and limp.