diphthong (n.) Look up diphthong at Dictionary.com
late 15c., from Middle French diphthongue, from Late Latin diphthongus, from Greek diphthongos "having two sounds," from di- "double" (see di- (1)) + phthongos "sound, voice," related to phthengesthai "to utter a sound, sound, raise one's voice, call, talk," which Beekes reports as of "no certain etymology. None of the existing connections with semantically comparable words ... is phonetically convincing."