Wikipedia: The earliest known recorded observations of Mercury are from the Mul.Apin tablets. These observations were most likely made by an Assyrian astronomer around the 14th century BC. The cuneiform name used to designate Mercury on the Mul.Apin tablets is transcribed as Udu.Idim.Gu\u.Ud ("the jumping planet"). Babylonian records of Mercury date back to the 1st millennium BC. The Babylonians called the planet Nabu after the messenger to the gods in their mythology.
The ancient Greeks of Hesiod's time knew the planet as Stilbon, meaning "the gleaming", and Hermaon. Later Greeks called the planet Apollo when it was visible in the morning sky, and Hermes when visible in the evening. Around the 4th century BC, Greek astronomers came to understand that the two names referred to the same body.
Personal Note: Had not known Mercury was known of that long ago. Learned something new today. Thank you.