Definition of buttock:
		
		
		(n) :
		
		(usually in the plural) Each of the two large 
fleshy halves of the 
posterior part of the body between the base of the back and the top of the legs
		
 
								
				"the headmaster prided himself on his 'fair' canings on bumbling boys' bare buttocks, administered alternatively from the left and the right, leaving each buttock equally painfully marked."