Generally refers to the decorative double stitching on the back pockets, shaped like bat wings. The arcuate is thought to be the oldest clothing trademark still in use.
Levi’s is credited as the first known brand to use of the design on their very first blue jeans in 1873. However, it has been speculated that they weren’t the inventors of the design.
However, arcuates are commonly associated with Levi’s. In goes further than mere association – particularly in the U.S., where no other denim brand is allowed to sell jeans with patterns that even remotely resemble the Levi’s arcs. Japanese reproduction brands have imitated the arc’s, which has resulted in several lawsuits.
Before Levi’s patented their seagull-arcuate in 1943, and before the introduction of Wrangler’s “Western Wear” W’s and Lee Lazy-S, all brands used the same design.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary and Denimhunters