gum·shoe 
/ˈgʌmˌʃu:/ noun
plural gum·shoes /ˈgʌmˌʃu:/ noun [count] US, informal + old-fashioned : a person whose job is to find information about someone or something : a private detective — used especially to imitate the language of old detective novels ▪ The movie's main character is a hard-boiled gumshoe.
