Seiti comes from a romanized corrupted version to portuguese (I´m brazilian) of my japanese name:

清一 

In fact, my 'real' name is Sei-iti.

It is a two parts name. The first half, 'sei', comes from the fact that every male member of my family (father, uncle, grandfather) has that initial on their own names. The other half, Iti/ichi, it´s because I´m the first born son.