ya mam is what people commonly known in great britan as 'chavs' say .If a individuale ever says this to you they are impling they want to have sexual intercourse or any other form of sexual activity with your mother.and is mostly a certain type of peoples (chavs) social word.
this is to 'chavs' one is asking the question and the other is repling by using the word 'ya mam'.
q) what are you doing tommorow
a) ya mam
this is usually followed (if in a group) by laughter as to certain individuals it appears funny.