This is a traditional Muslim greeting which means “peace be unto you.” The traditional response is wa alaikum assalam, which means “and on you be peace.”

Their is 2 ways in saying it – the mockery way the real way I'm not extreme but because of the stories of our past from the days off the Christians/jew Arabs they used to use the word as-sallam which isn't like alsallam , al is a sign of respect for introductory and the phrase "a" followed by a long connecting vocal is an annotated response in Arabic which is reversed this is how as-sallam allaikom looks in arabic
أسَّلام عليكم I dont know how to explain how it means in english but its close to ..'does peace ever be with u' its like a mockery in other words in the end its the intentions u would say of the person but I know one clown has this embedded in he's genes how you reply to them is easy just by saying wallaikom meaning and same to u.