Mariposa is the Spanish word for 'butterfly', and also the Portuguese word for both 'butterfly' and 'moth'; in Portuguese it is much more common to call a butterfly a borboleta.
It is generally held that this word comes from the the phrase Mari, posa(te), literally "Mary, alight!", a phrase that was used in children's songs and games. A rather less popular theory is that it comes from male pausat, meaning "badly alighted".
While the word has not really entered the English language, it does appear frequently in names of businesses, towns, and ships, among all those other places where an odd word with nice connotations is desirable.
In Spanish Mariposa is also derogatory slang for a gay man, and in Portugal, slang for a prostitute. So be careful what you name your boat.