IPA: /kəʊˈɪn.sɪ.dənt.li/
KK: /kəʊˈɪnsɪdəntli/
Happening at the same time as something else, often by chance.
They arrived at the party coincidently, not knowing each other would be there.
Coincidently → It is formed from "co-" (meaning together) and "incidere" (meaning to fall upon or happen). The word describes events that happen together by chance or simultaneously.
Think of things happening 'together' ('co-') by 'chance' ('incidere') — that's what coincidently means.