What is the meaning of the word "casual"?

Definitions:

appropriate for ordinary or routine occasions adjective
- casual clothes
characterized by a feeling of irresponsibility adjective
- a broken back is nothing to be casual about
marked by blithe unconcern adjective
- an ability to interest casual students
natural and unstudied adjective
- using their Christian names in a casual way
not showing effort or strain adjective
- a difficult feat performed with casual mastery
occurring from time to time adjective
- casual employment
occurring or appearing or singled out by chance adjective
- seek help from casual passersby