It was a mispronunciation of a European (i.e. German/Old English) spelling of Jesus. Before the 1600s "Js" were pronounced "Y" - as in the name Jürgen. In fact, the letter "J" is extant in no alphabet until the 14th Century. The Greek Ίησους (Yeh-sus) was written "Jesus", but pronounced the same (that is, with a "Y") until after the 1630s when the letter J was generally pronounced "dje". This accounts for the change in pronunciation.
This also accounts for why in the 1611 KJV, all the "J - names" (e.g. Jesus, James) were written with an "I" (e.g. Iesus, Iames).
If you're interested, the English/German form is actually taken from the Latin Iesus, hence the "us" ending, rather than a "ous" ending. The name in Hebrew is ישוע (Yeshua, a shortened form of Yehoshua), which is where we get the name Joshua from. :-)