This sound is an unaspirated ch, just as d and g represent unaspirated t and k.
1725, "sound of the letter 'H'," especially at the beginning of a word, from Latin aspiratio "a breathing, exhalation, the pronunciation of the letter H" (see aspire).
"to pronounce with audible breath," 1700; perhaps a back-formation from aspiration (n.2), or from French aspirer (1520s), or directly from Latin aspiratus, past participle of aspirare (see aspire). Related: Aspirated; aspirating.
aspirate as·pi·rate (ās'pə-rāt')
v. as·pi·rat·ed, as·pi·rat·ing, as·pi·rates
To take in or remove by aspiration. n. (-pər-ĭt)
A substance removed by aspiration.