small-minded person; person of poor character
Colloquial, mildly derogatory phrase for someone petty, narrow-minded, gossipy, or lacking in character. Often used in casual criticism.
Getting angry over something like that—what a small-minded person.
Always badmouthing others is what petty people do.
From ちっちゃい (colloquial variant of 小さい 'small') + 人間 'human being'. The phrase figuratively describes a person of small character.