IPA: /ˈdʒɔɪntli/
KK: /ˈdʒɔɪntli/
In a way that involves two or more people or groups working together.
The two companies decided to invest jointly in the new project.
Jointly → It is formed from "joint" (from Old French *joint*, meaning joined or connected) and "-ly" (meaning in a manner). The word "jointly" means in a manner that is joined or together.
Think of things that are 'joined' together — that's what 'jointly' means, doing something together.