IPA: /ˌoʊvərˈkɒnfɪdəntli/
KK: /oʊvərˈkɒnfɪdəntli/
In a way that shows too much confidence, often without justification.
He overconfidently claimed he would win the race without even training.
Overconfidently → It is formed from "over-" (meaning excessively) and "confident" (from Latin "confidere", meaning to trust or have faith) and the suffix "-ly" (meaning in a manner). The word describes acting in a manner that is excessively trusting or self-assured.
Think of being 'excessively' ('over-') 'trusting' ('confidere') in your abilities — that's why acting overconfidently means being too sure of yourself.