I love learning the origins of common English words and phrases.
Hello apparently comes from hallo which was used to incite hunting dogs and is relatively recent (mid 1800s). It became popular after the advent of the telephone, although Alexander Graham Bell originally suggested Ahoy as a greeting!
Hi doesn’t actually come from Hello. It was first record in the 1860s being used by a native American from Kansas and is thought to be a variant of the middle English word hy or hey which/is was used to attract someone’s attention.
Goodbye, on the other hand, is much older (16th century) and came from an abbreviation of ‘God be with ye’ – it would have been written Godbwye which, over time, became, Goodbye!
Bye-bye also doesn’t come from goodbye but was used in the nursery to lull babies to sleep.
Information from dictionary.com / Wikipedia /Cambridge Dictionary