'slap' originates from imitative/echoic English usage (echoic origin), probably attested in Middle English as 'slappen' representing the sound of a sharp blow.

'slap' changed from Middle English 'slappen' (and may be related to similar imitative forms in Old Norse or other Germanic dialects) and eventually became the modern English word 'slap'.

Initially it referred directly to the sound and action of a quick sharp blow; over time the primary sense of striking remained but extended metaphorically (e.g. 'a slap in the face') and, in modern slang, to positive intensity when applied to music ('this track slaps').