Slap echo or slapback is a single echo resulting from non-absorbing (i.e., reflective) surfaces, characterized by a significant amount of high frequency content. So-called because you can test for slap echo by sharply clapping your hands and listening for the characteristic sound of the echo in the midrange. Slap echo can smear a stereo sound field by destroying the critical phase relationships necessary to form an accurate sound stage. Devices that simulate slap echo are popular in recording. One distinct repeat-echo is added to an instrument sound resulting in a very live sound similar to what you would hear in an auditorium. The echo may be placed in a different spatial location in the stereo mix. Normally, the echo delay is just large enough to be heard as a discrete echo on careful listening (e.g., on the order of tens of milliseconds - 40-120ms). Slap back is very popular in 1950s-style recordings such as "rockabilly tunes.
Source: Slap Echo Sweetwater