means Showing open defiance toward authority, rules, or convention — refusing to obey or conform.

from From Latin 'rebellis,' meaning 'one who renews war,' built from 're-' (again) and 'bellum' (war) — the same 'bellum' that gives us 'belligerent.' A rebel was literally someone who took up arms a second time, defying a peace already settled. The word marched through Old French 'rebelle' before settling into English, the '-ous' ending turning the rebel's act into a lasting temperament.