http://www.etymonline.com/index.php?term=Jehovah
The Encyclopedia Britannica (Micropedia, vol. 10) says: "Yahweh-the personal name of the [El] of the Israelites ...The Masoretes, Jewish biblical scholars of the Middle Ages, replaced the vowel signs that had appeared above or beneath the consonants of YHWH with the vowel signs of Adonai or of Elohim. Thus the artificial name Jehovah (YeHoWaH)came into being. Although Christian scholars after the Renaissance and Reformation periods used the term Jehovah for YHWH, in the 19th and 20thcenturies biblical scholars again began to use the form Yahweh, thus this pronunciation of the Tetragrammaton was never really lost. Greek transcriptions also indicate that YHWH should be pronounced Yahweh."
Dr. J. B. Rotherham states in the preface of his Bible concerning Jehovah: "Erroneously written and pronounced Jehovah, which is merely a combination of the sacred Tetragrammaton and the vowels in the Hebrew word for Lord, substituted by the Jews for JHVH, because they shrank from pronouncing The Name, owing to an old misconception of the two passages, Ex. 20:7 and Lev. 24:16...To give the name JHVH the vowels of the word for Lord [Heb. Adonai], is about as hybrid a combination as it would be to spell the name Germany with the vowels in the name Portugal - viz., Gormuna. The monstrous combination Jehovah is not older than about 1520 A.D."