Origin of the word H

1. the pronunciation "aitch" was in O.Fr. (ache), and is from a presumed L.L. *accha (cf. It. effe, elle, emme), with the central sound approximating the value of the letter when it passed from Roman to Germanic, where it at first represented a strong, distinctly aspirated -kh- sound close to that in Scottish loch. In earlier L. the letter was called ha. In Romance languages, the sound became silent in L.L. and was omitted in O.Fr. and It., but it was restored in M.E. spelling in words borrowed from O.Fr., and often later in pronunciation, too. Thus Mod.Eng. has words ultimately from L. with missing -h- (e.g. able, from L. habile); with a silent -h- (e.g. heir, hour); with a formerly silent -h- now vocalized (e.g. humble, honor); and even a few with an excrescent -h- fitted in confusion to words that never had one (e.g. hostage, hermit). Relics of the formerly unvoiced -h- persist in pedantic insistence on an historical (object) and in obs. mine host. The use in digraphs (e.g. -sh-, -th-) goes back to the ancient Gk. alphabet, which used it in -ph-, -th-, -kh- until -H- took on the value of a long "e" and the digraphs acquired their own characters. The letter passed into Roman use before this evolution, and thus retained there more of its original Sem. value.

via Online Etymology Dictionary, ©2001 Douglas Harper