According to Eucharistic Prayer I, Jesus took “this” cup in his hands. Why this cup? There was an early controversy concerning whether water might be used instead of wine in the Eucharist. The wine-water controversy provides the key to why the Roman canon says “this potent [i.e. alcoholic/spiritually inebriating] cup”—meaning wine. The vessel is plainly not the focus; its contents are. In English, the word “cup” can refer to both the vessel and what it contains. Not so “chalice.” You can drink from a chalice, but you cannot “drink a chalice.” If you are talking about the contents of the vessel, “cup” is the only word that works.