匪 (fěi) is a bound form (不能单独使用, must combine with another morpheme) meaning 'bandit' or 'robber'. You'll find it in compounds like 土匪 (local bandit), 匪徒 (gangster), and 绑匪 (kidnapper). It cannot stand alone as a noun in modern Mandarin—always use it as part of a multi-character word.