sleeper wave
n.— «A massive wave—known to locals as a sleeper wave—crashed onto shore, taking Mikayla. Pettenger reached the girl first, grabbing her fleece pullover jacket. She lost her grip, but grabbed Mikayla again.» —“Waves claim two at beach” by Ryan Lillis, Christina Jewett Sacramento Bee (California) Oct. 25, 2006. (source: Double-Tongued Dictionary)