Smiley

From Matronics

A smiley is an indentation in the surface of the skin of a riveted aircraft that is caused by the lip of a standard rivet die contacting the skin surface during riveting. This usually results from the die not being perpendicular to the surface while the rivet is set.

The term derives from the curved shape of the indentation, which is similar to the smile on a happy face or the parenthsis character on an ASCII happy-face emoticon. To the builder, this resembles a sardonic sneer more so than a happy smile.