The word comes from the Hebrew "יְהוָה", which is what Moses said after God revealed himself to Moses.
Let's look at the verses where this word appears:
"And to Seth, to him also there was born a son; and he called his name Enosh; then began men to call upon the name of the LORD."
(Genesis 4:26)
"And God said unto Moses: 'I AM THAT I AM'; and He said: 'Thus shalt thou say unto the children of Israel: I AM hath sent me unto you.'
And God said moreover unto Moses: 'Thus shalt thou say unto the children of Israel: The LORD, the God of your fathers, the God of Abraham, the God of Isaac, and the God of Jacob, hath sent me unto you; this is My name for ever, and this is My memorial unto all generations."
(Exodus 3:14-15)
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These are the earliest appearances of the word. It is translated to "LORD", this is the word they used in Hebrew to describe God in a personal manner.
The word itself "יְהוָה", and the translation to "Yahweh", or more accurately "Jehovah", is the TRANSLITERATION of the word. For example, in Islam, "Allah" really means God, but it's transliterated as "Allah" for us English speakers.
That's it. It's a transliteration of the Hebrew word, which is the name of God.
Why this name? Well, I'm not sure, but it could mean something other than we think, let me explain. I know some Hebrew, and I know the word "to be" is "Lehiot", and "was" is "Haia", and "to be" is "Yehieh", and as we know nobody speaks ancient Hebrew anymore, so I think the origin of the word is something like what God told Moses in Exodus as we see above, that his name is simply "I AM", or "HE IS", "TO BE". Something along those lines, and since we can't pronounce it today, we transliterate it to "Yehovaa" or "Jehovah".