A 'Phlatty' is a term to describe a marijuana blunt that after being rolled is 'flat' instead of 'round'. Common with inexperienced blunt rollers. Most commonly occurs when incorrectly sealing the blunt paper back together as some people will use to much pressure and/or etc., thus creating a flattened out blunt rather than a cylinder shaped blunt.
Yo I just rolled that blunt but it came out to be a phlatty.
Iv'e been rolling them phlatty's all day son!