An error of transcription made when translating a pointed (vowelled) Hebrew Bible into English (or perhaps Latin. I don't know if St.Jerome made this error).
In the Hebrew Bible YHWH (the tetragrammaton, believed to be a noun derived from a verb 'to exist', hence, 'I Am that Exists', colloquially 'I Am What I Am') is traditionally unpointed. However, Hebraic tradition also forbids the pronunciation of this word with it's proper pointings (which are a long a (aah) and a flat a (ah)), resulting in Yahweh.
To compromise, Jewish teachers pronounce the word 'adonai', meaning Lord, instead of Yahweh. To reduce confusion, the tetragrammaton was pointed with the vowel symbols for adonai, which in Hebrew results in an impossible word, Y^eho^wa (it is difficult to show with the Roman alphabet, but trust me, it's impossible to do in Hebrew). This is what was translated by misunderstanding scholars as Jehova, the Lord.