IPA: /dɪˈsaɪdɪdli/
KK: /dɪˈsaɪdɪdli/
In a way that is clear and certain, leaving no doubt about something.
She was decidedly happy with the results of her exam.
Decidedly is formed from "decidere" (meaning to cut off or determine) and the adverbial suffix "-ly" (meaning in a manner of). The word means in a manner that is determined or definite, indicating a clear and firm decision.
Think of 'deciding' as 'cutting off' options, and the '-ly' shows how you do it. So, when something is decidedly done, it is done in a clear and definite way.