A reference to a zen koan, worn threadbare and meaningless by endless repetition in the media. It has no 'answer' unless, as in definition (1) above, the answerer redefines the term 'clapping', thus entirely missing the point.
The sound of one hand clapping is supposed to be a question with no answer. It is a clasic Zen koan given to students to aid in enlightenment by leading the mind into a cul-de-sac with no way out but enlightenment. How enlightened could the person who came up with this koan be, considering it has such an obvious and simple answer?
Fundamental lore of Zen Buddhism; know as the Kōan traditional. In the west formally termed onanism or colloquially: ‘throttling the purple headed yogurt slinger’. This is the sound of skin slamming skin as “mediation” reaches the point of enlightenment.
“If you’re doing nothing, why can half the building hear the sound of one hand clapping? I swear if you’ve ruined another one of my Grattan catalogues with your man milk I promise you this is the last time Madame palmer's five daughters are coming 'round to play.”