If you're mannerly, you're extremely well-behaved and polite. You might be so mannerly at your best friend's house that your parents wouldn't even recognize you!
Mannerly is another way to say "well-mannered." If you're mannerly, your manners are impeccable — you follow all the appropriate rules of etiquette. While mannerly people keep their elbows off the table and their napkins in their laps, people who are not mannerly might talk loudly with their mouths full or eat off their neighbors' plates.