In heraldry and architecture, a line which is drawn nebuly (or nebulée) is made up of a series of bulbous protrusions, which are supposed to resemble clouds. The term is derived from the Latin word nebula, "a mist, vapor, or cloud"(OED). A nebuly line meanders in one direction describing the shape of three-quarters of a circle, then similarly in the other direction, maintaining a straight overall trajectory. A fess nebuly is a fess where the two horizontal lines of the fess are nebuly. A shield formed of several such fesses, termed bars where in excess of one, is barry nebuly.
The Collaborative International Dictionary
Nebuly \Neb"u*ly\, n. (Her. & Arch.) A line or a decoration composed of successive short curves or waves supposed to resemble a cloud. See N['e]bul['e]
N'ebul'e \N['e]`bu`l['e]"\, Nebuly \Neb"u*ly\, a. [F. n['e]bul['e].] (Her.) Composed of successive short curves supposed to resemble a cloud; -- said of a heraldic line by which an ordinary or subordinary may be bounded.
a. (context heraldry English) Edged in a deeply wavy line, intended to represent clouds.