IPA: /ˌæntɪˈmɒnəˌkɪst/
KK: /æntɪˈmɒnərkɪst/
Not in favor of a system of government where a king or queen rules.
The antimonarchist group organized a rally to promote their views on democracy.
A person who does not support or is against the system of monarchy.
The antimonarchist group organized a protest to advocate for a republic.
Antimonarchist is formed from "anti-" (meaning against) and "monarch" (from Greek "monarchēs", meaning ruler or sovereign) and the suffix "-ist" (meaning one who practices or advocates). The word describes a person who is against monarchy or the rule of a monarch.
Think of someone who is 'against' ('anti-') a 'ruler' ('monarch') — that's why an antimonarchist opposes monarchy.