zwyczajnie in English:

Jill is dressed casually

She glanced casually through a magazine as she waited.

if people dress casually, they wear comfortable clothes that are suitable for informal occasions

She dresses casually at the weekends.

she casually asked about your girlfriend

He picked up the newspaper and glanced casually over the front page.

He was an amiable fellow, but had a certain reserve that he would not let you casually trespass upon.