In surfing slang terms, what is an 'ankle slapper'?

Ankle Slappers are known as Small waves. They are waves that barely go higher than the surfers ankle.

Surf-speak has evolved over the decades and is forever changing as new generations of coastal locals and traveling surfers rise. Surfer slang is tossed around in the lineup, on the sand between sessions, or inside the local surf shop. Some terms stick around while others fade and more than likely you’ve heard surfer slang in movies such as the 60s surf film "Endless Summer", "Fast Times at Ridgemont High", or the 1991 action movie, "Point Break" with Keanu Reeves playing an undercover surfer who undermines surf rebel, Patrick Swayze.

Surf slang is unique because it derives from the personal stoke of surfing. The word, stoke, or stoked, meaning fired-up or excited, derives from the act of harnessing water beneath your feet while charging a big wave. A feeling only a few people experience in their lifetime, and a feeling that has altered the human language worldwide. Hardly any other extreme sport or action activity carries such a lengthy and well-known vocabulary as surf slang does.