Figure of diction, as peers say, very common in the evolution from Latin to Spanish consisting of the addiction of some intervocical phoneme in words to facilitate their phonetics or for some other reason. The etymology says epi ( envelope ), in ( in ), thesis ( position ) : get in the middle. An example: Hóminem, accusatory of homo, lost the final m and was in hómine. The loss of the i gave homne. But two nasal consonants together did not convince and the n became r remaining homre. The epintesis arrives and gives us man. Well, man! Look how good it is!