From Gardiner, The Inscriptions of Sinai, Part I

This short Egyptian hieroglyphic inscription from Serabit includes five examples of a figure with two raised arms (highlighted in orange), This sign was possibly used to designate the Egyptian word for “foreman.” Goldwasser believes that the Canaanite inventors of the alphabet, who must have associated this sign with the foreman’s loud calls to work (Hoy!), modeled both the shape and phonetic value of the Proto-Sinaitic letter “H” after this sign.