"morte moriatur" -Ex 22:19It is the Vulgate translation of my favorite Hebrew idiom which is the infinitive absolute plus the verb to illustrate necessity, so from the Hebrew it means "he must die." St. Jerome though would find this too literal a translation being that he does not translate it that way, but rather "in death let him die." (It might have been better to translate it My favorite way to translate its some where in between "dying he must die." It just sounds more cool.
The Vulgate actually does not translate most infinitive absolutes this way, it usually (although I only looked at the first 20 or so examples out of several hundred) just uses a single verb in the indicative or subjunctive depending on context.