goodbye

Từ điển American Heritage Dictionary 4th

good·bye or good-bye also good-by (go͝od-bīʹ)interj.
Used to express an acknowledgement of parting.n. pl. good·byes, also good-·bys
1. An acknowledgment at parting, especially by saying “goodbye.”
2. An act of parting or leave-taking:
many sad goodbyes. [Alteration (influenced by good day), of God be with you.] 
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.