…et cum Spiritu Tuo.
This reinstitution into the Mass is a primary example of a goal of the new translation: a precise, historical, and Biblical transformation of the Latin into our modern language. St. Paul makes use of this phrase in his letters to Philemon and Timothy. In addition, it has been used by the East and the West from the earliest days of the Church, as mentioned by St. Hippolytus around the year 215 A.D.
Today, English is the only major language of the Roman Rite which did not translate the word spiritu. The Italian (E con il tuo spirito), French (Et avec votre esprit), Spanish (Y con tu espíritu) and German (Und mit deinem Geiste) renderings of 1970 all translated the Latin word spiritu precisely.
The expression “et cum spiritu tuo” is only addressed to an ordained minister. The reference to "your spirit" can therefore be contemplated as a reference to the indwelling gift of the Spirit: precisely that Spirit poured out at ordination, and precisely the very same Spirit through which the Holy Sacraments are to be performed. In their response, the people assure the priest of the same divine assistance of God’s spirit and, more specifically, help for the priest to use the charismatic gifts given to him in ordination and in so doing to fulfill his prophetic function of the Church in persona Christi.
for more info, see http://www.usccb.org/romanmissal/