'to be six of one and half a dozen of the other', where something is referred to differently but is essentially the same thing, for instance, one person may refer to a trolly dolly as an air stewardess or an air hostess or as to borrow from the Americans, a flight attendant. Different words describe the same job. There is also the saying 'to be at sixes and sevens'.
Only one of these three actually starts with 'six', but what the heck! 'Six feet under', meaning 'dead and buried', based on the standard depth of a grave. To 'deep six' something means to get rid of it, generally by putting it where it won't reappear...a reference to six fathoms of water.To 'knock for six', meaning to astonish/affect badly...a reference to a major 'hit' at cricket.