Hello (first attested in 1833), from holla, hollo (attested 1588). This variant of hallo is often credited to Thomas Edison as a coinage for telephone use, but its appearance in print predates the invention of the telephone by several decades.
From Wiktionary
Alteration of hallo alteration of obsolete holla stop! perhaps from Old French hola ho ho! la there (from Latin illāc that way)
From American Heritage Dictionary of the English Language, 5th Edition
Possibly from the Old High German and Old Saxon verb halon, holon (“to bring something, to gather”), akin to English hale or hail.
From Wiktionary