lip
The upper edge of a wave; are part of the wave that steepens, fringes, and spills over. Although surfers regularly do off-the-lip maneuvers on soft, crumbling, easy-breaking waves, the expression "lip," used alone, generally refers to the cresting portion of a tubing wave.
Two surf movies, Hot Lips and Inner Tubes (1975) and Wet Lips (1983), turned the word into an all-too-easy sexual double entendre. Similar to "curl."