Extended Strong's Greek Lexicon (Abbott-Smith based)
fool
Usage: ῥακά (T, ῥαχά), usually taken to represent the Aram. רֵיקָא, a shortened form of H7386, " empty," as vocalized in the Galilæan dialect; an expression of contempt, raca: Mat.5:22 (cf. DB, iv, 191 f.; and for other explanations, see Zorell, see word).† (AS)