Namu means "Devotion" and comes from the sanscrit "namas." It can be debated that it is etymologically related to the English word "Name" to "Numeration" and to other roots related to declarations and namings through the "indo-european" connection. It is the word that "activates" the Lotus Sutra in people's lives. And it is a word of "esoteric" power. In India women often put a mark where their third eye would be, that mark is known as a "Namas" mark and indicates their devotion to their husband. Namu "opens" Myoho Renge Kyo just as the Namas mark is located where the "third eye" is. Faith is the "evidence of things unseen" and devotion is the creative application of faith to life.
Myoho means "mystic dharma" or "mystic law" and refers to the inexhorable laws of life. The "mystical" is deep, unfathonable, beyond our ken. And "laws" are things that operate whether we like them or not. While human laws are meant to govern us, discipline us, and help us navigate the world of natural laws and relationships.
Renge is the Lotus Flower, and is also "causality". The Lotus Flower manifests its seeds along with the flower.
Kyo, is the translation of "sutra" and also means "harmoneous sound." It is the sounds of the universe that manifest its power.