goodbye
I was wondering what part of speech "bye" is; my smart friend correctly guessed interjection, even though it doesn't seem much like an interjection to me (although, really, what else could it be). Dictionary.com also gave the origin (1565-75; contr. of God be with ye) and the following helpful paragraph, which they apparently got from The American Heritage® Dictionary of the English Language:
Word History : No doubt more than one reader has wondered exactly how goodbye is derived from the phrase "God be with you." To understand this, it is helpful to see earlier forms of the expression, such as God be wy you, god b'w'y, godbwye, god buy' ye, and good-b'wy. The first word of the expression is now good and not God, for good replaced God by analogy with such expressions as good day, perhaps after people no longer had a clear idea of the original sense of the expression. A letter of 1573 written by Gabriel Harvey contains the first recorded use of goodbye: "To requite your gallonde [gallon] of godbwyes, I regive you a pottle of howdyes," recalling another contraction that is still used.
Word History : No doubt more than one reader has wondered exactly how goodbye is derived from the phrase "God be with you." To understand this, it is helpful to see earlier forms of the expression, such as God be wy you, god b'w'y, godbwye, god buy' ye, and good-b'wy. The first word of the expression is now good and not God, for good replaced God by analogy with such expressions as good day, perhaps after people no longer had a clear idea of the original sense of the expression. A letter of 1573 written by Gabriel Harvey contains the first recorded use of goodbye: "To requite your gallonde [gallon] of godbwyes, I regive you a pottle of howdyes," recalling another contraction that is still used.