Other forms: goodbyes
Goodbye is what you say when you leave, or when somebody else does. It is the neutral, complete form — safe with anyone, from a stranger to a grandmother.
Everything else in this family is a register choice. Bye is friendly and everyday. Bye-bye is for small children, or for adults being playful. Farewell is literary and final. See you is casual and assumes there will be a next time; goodbye alone can sound heavier than you meant, which is why the end of a phone call is usually bye rather than goodbye. As a noun it takes a plural — long goodbyes, saying our goodbyes — and to say goodbye to something means to give it up for good.
1phrasegoodbyeWhat you say when you or somebody else is leaving.
“She said goodbye at the door and drove off.”
2nounactAn act of saying goodbye.
“It was a long goodbye at the station.”
Too few real examples of this word to estimate which sense is commonest.
goodbye is a parting word.
Also accepted in games: bye, farewell
Goodbye is a worn-down blessing: God be with ye, said fast for four hundred years until the words fused. The good came from the pattern of good day and good night, which is why it no longer looks like God at all.
Proto-Indo-European
*ǵʰew-
English, today
goodbye
From God. From earlier goodby, Godby, Godby'e, Godbwye, God b'w'y, God bwy yee, God buy you, God be wi' you, each a progressively shorter contraction of God be with ye or God be with you. Note the change of God to good by confusion with good morning, good night, etc. Modernly analysed as good + bye.
Etymology from Wiktionary, CC BY-SA