The story
Origin uncertain; probably of Angloromani origin. Compare Romani chavi (“male child”) or ćhavo, shavo (“female child”), chal (“boy”), chavvy (“mate, friend”), compare Swedish tjej; possibly cognate with Portuguese chavalo, Spanish chaval, German Chabo, Russian чувак (čuvak), Hungarian csávó. See also charva. Derivations from council-housed and violent, Cheltenham average (and similar) are folk-etymological backronyms.
Etymology from Wiktionary, CC BY-SA 4.0.