This is a Hebrew word that derives from a very common root: aman, which means "to be firm", "stable", "to have trust", "to believe firmly". The word amen occurs fourteen times in the Torah as a concluding formula, or in response to prayer.
In the language of prayer, but not only there, by pronouncing amen (with the stress on the final syllable), one firmly declares one's agreement with what has been heard or repeated.