A fiddleback waist is a beveled waist with a pointed, fishtail-like shape where it mees the forepart. Fiddleback waists are commonly accentuated by contrast coloring of the waist and forefoot of the sole.
A similar effect is sometimes created by inserting a wedge-shaped insert beneath the outsole through the waist, then hammering the outsole extensively to take the shape.
Some credit Gaziano & Girling with popularizing the feature most recently.