"Chalk It Down" is generally used by stoners in Ireland, it can mean various things, too many to even list but people use it as a substitute for "chill" or "ok" or "in your face"
Girlfriend: Are you messing around with Lisa???!!!
Boyfriend: Whoah Chalk it down! (chill)
Boy1:You coming to the party tonight?
Boy2:Ya, i'll be there @ 9, chalk it down! (ok)
Boy:I fucked your 'ol doll
last night, ha chalk it down! (in your face)