The etymology of mumpsimus (if you can call it that)
The word mumpsimus derives from this prayer from the Roman Mass. This prayer, along with the following corpus tuum, Domine, comprise the priest's silent thanksgiving after having communed. If he has said Mass with a congregation present, he will recite these words after distributing Communion (Eucharist) to the faithful.
Quod ore sumpsimus, Domine, pura mente capiamus: et de munere temporali fiat nobis remedium sempiternum.
A more literal translation might be as follows, with inferred parts from the Latin in quotes:
What we have taken by mouth (as food i.e. the Eucharist), O Lord, may gain us a pure mind: and let eternal healing be (given) to us from the gift of time. (my translation)
Like the royal 'we' the 1st person plural applies to the priest alone.
Although illiteracy cannot be explained by recieved text, liturgical textual errors frequently do not, as in this case, result in grammatical error. This phenomenon is evidenced by the 1560 Latin Book of Common Prayer where the concluding phrase of the Lord's Prayer, sed delivera nos a malo replaces livera for delivera. Organic changes in liturgy from city to city and town to town complicate liturgical philology, given that Roman liturgy solidified only at the Council of Trent and only in response to Protestant erosion of Catholic supremacy.