He just didn't have the philosophical or theological equipment to do better than 'eucharisted'. Since then, the bloom of the Eucharistic mystery has opened up much more, and St Thomas Aquinas was able to define it, so that future Christians could watch more petals unfold. Other doctrines which show the effect of this development are some concerning Mary, the Mother of God. We will look at those in Leaflet 16. This is what the Church calls 'papal infallibility'.
Infallibility is greatly misunderstood.
People seem to think it means that the Pope will never make a mistake at all, or that he will never sin. This isn't the case - infallible means 'will not fail'. It doesn't just apply to the Pope. Since Jesus promised to be with His Church for all time, Catholics believe infallibility applies to the whole Church in particular ways. In other words, Jesus Himself willed that in order that people not be misled, He would grant the Church a share in His own unfailing Truth.