Full Definition of INFEST. transitive verb. 1. 2. : to live in or on as a parasite. — in·fes·tant \-ˈfes-tənt\ noun. — in·fes·ta·tion \ˌin-ˌfes-ˈtā-shən\ noun. — in·fest·er \in-ˈfes-tər\ noun. See infest defined for English-language learners.ull Definition of INFEST. transitive verb. 1. 2. : to live in or on as a parasite. — in·fes·tant \-ˈfes-tənt\ noun. — in·fes·ta·tion \ˌin-ˌfes-ˈtā-shən\ noun. — in·fest·er \in-ˈfes-tər\ noun. See infest defined for English-language learners.