The Greek adjective akataschetos (ἀκατάσχετος) means that which cannot be held back, unrestrained, or unable to be controlled — composed of the alpha-privative and a form of kataschō (to hold down, restrain).
James 3:8 uses this word: no human being can tame the tongue — it is restless, unruly evil. This is not pessimism but realism about human fallenness. Only the Spirit of God can truly tame what is inherently unruly in the fallen heart. This points to the necessity of regeneration — the inner transformation that produces a transformed tongue (Matthew 12:34).