Amen אָמֵן derives from a triliteral root meaning 'to be firm', 'confirmed', 'reliable', 'faithful', 'to have faith, believe'. It has a number of lexical implications: 1. reliability, sureness; 2. stability, continuance; 3. faithfulness, reliability: (a) of men, (b) an attribute of God; 4. truth (a) as spoken, (b) of testimony and judgment; (c) truth as a body of ethical or religious knowledge; 5. adv. in truth, truly. When used liturgically or in response to a statement it means something along the lines of "so be it" (= it is trustworthy).
Either Sadhu or Metta would perhaps seem to fit the bill quite well?