The first Council of Nicaea, meeting in 325, gave a ruling about the dying which is short and unusually firm. Anyone at the point of death, it says, is not to be deprived of the last and most necessary viaticum. The force of it lies in the context. The early church's penitential discipline could exclude a person from communion for years, and the canon says that the discipline stops at the deathbed: whatever a person is under, they receive. A system of exclusion is thereby made to answer to something it cannot override.
The name carries the same idea in a different image. Viaticum in ordinary Latin is what a traveller takes for a journey, the provision put up for the road, and its use for communion given to the dying is a plain metaphor that the tradition has never felt the need to elaborate. It is the same sacrament as any other communion, brought to a bedside rather than received in church, and the rite around it is short: a greeting, a reading, the creed if the person can say it, the Lord's Prayer, and the giving of communion with words that name the journey.
Reservation of the sacrament outside the liturgy grew up largely to make this possible. Bread cannot be consecrated at a moment's notice at three in the morning, so churches kept some. The Byzantine practice of preparing a Lamb on Holy Thursday, drying it and keeping it through the year for the sick and the dying is the eastern form of the same provision, and it is the reason the reserved sacrament exists in that tradition at all.