What is Mannerism?

my grandaughter has been diagnosed with having a mannerism she is 18 months old any ideas

To have manners - NO that is to be well mannered! (don't know who said mannerism is to have manner especaily sinnce you can't be diagnosed with "having manners,") _AsdogGeek

A mannerism is a behavior or habit that a person keeps on doing without even thinking about it,"

one example given is har twirling, lip chewing rocking and hand flapping may also be mannerisms- ASDogGeek