A Fly Leg which is not Straight and dips toward Rod Leg.

Related terms in Fly Leg Shapes: Straight Fly Leg, Convex Fly Leg, Trailing Loop, Tailing Loop, Under-Running Loop, In-Plane Loop

Practise it: What causes a tailing loop, and how do I fix it?

From the FFI Fly Casting Definitions (2025 Edition), reproduced with attribution to Fly Fishers International.