English speakers use mouth for the opening through which we eat and speak, and they stretch the same word to river outlets, bottle rims, cave entrances, and anyone who talks too much. Those extensions feel natural because the core image is an opening that takes things in or lets things out. The origin of the word mouth is easiest to see when English is set beside its Germanic relatives: the anatomy sense is old, the extra senses grew from it, and the spelling still hides a lost consonant.
A native word, not a loan
Mouth did not arrive with French after the Norman Conquest or with Latin through the schools. It is inherited. Old English already had mūþ for the oral opening of a person or animal and, more loosely, for a door, a gate, or any opening. Middle English kept it as mouth, mouþ, and nearby spellings. From there the modern form is a matter of sound change, not of borrowing.
The immediate ancestor is Proto-West Germanic *munþ, itself from Proto-Germanic *munþaz. That reconstruction does not rest on English alone. Gothic has munþs. Old Norse has munnr, continued in Icelandic munnur, Norwegian munn, Swedish mun, and Danish mund. Old High German munt becomes German Mund. Dutch has mond. West Frisian has mûn. Scots has mooth. Across early Germanic the word is a masculine a-stem noun meaning mouth.
Why English lost the n
The English form looks slightly odd beside German Mund and Dutch mond, which still show an n. The n was once there in the English line as well. In the Ingvaeonic languages — Old English, Old Frisian, and Old Saxon — a nasal consonant regularly dropped before a fricative such as þ, and the preceding vowel lengthened to compensate. Proto-Germanic *munþaz therefore yielded Old English mūþ, with a long ū and no n. The same change produced English tooth beside forms that kept a nasal, five beside German fünf, goose beside Gans, and us beside uns.
Old Saxon later restored the nasal in many words, which is why Low German ended up with Mund again. Coastal Dutch keeps a related remnant in muide and mui for a tidal inlet, even though standard Dutch mond has the n. English never put the n back. The long ū of Old English mūþ later diphthongized under the Great Vowel Shift, giving the modern vowel of mouth. The verb, written the same way, usually voices the last consonant, so to mouth something ends in a sound like the end of bathe. That pairing of a voiceless noun and a voiced verb is an English habit, not a separate etymology.
There is no prefix or suffix to unpack. Mouth is a simple inherited stem. The verb is later and secondary: Middle English mouthen, from the noun, is recorded from the early fourteenth century in the sense to speak. Old English had already used a related verb muðettan for blurting things out.
How far back the root goes
Beyond Germanic the trail is real but disputed, and the honest account stops short of a single tidy Indo-European verb. A western cluster of forms points to a reconstructed *mn̥t- or *mn̥-tó-s for a projecting part of the face. Latin mentum means chin. Welsh mant means jaw or mandible, sometimes mouth. Hittite mēni- has been compared in the sense face or cheek. Many handbooks treat this as a suffixed zero-grade of a root *men- meaning to project, the same family sometimes invoked for Latin eminere, to stand out, and for mentum as a jutting body part. On that view, Germanic shifted the sense from chin to mouth.
Not every dictionary is willing to lock that in. Some limit the etymon to Celtic, Italic, and Germanic, treat the Hittite link as only a suggestion, and note that taking *mn̥t- as a verbal adjective of a root meaning to stick out rests on a base that is thinly attested outside Latin. Others prefer a Proto-Indo-European *ment- for chewing, the jaw, and the mouth, and some scholars have tried to connect the word with Latin mandō and Greek masáomai, both meaning to chew. Those chewing links are themselves contested. A more speculative idea relates the Germanic noun to a longer root behind Greek stoma, mouth; that remains outside the usual account.
What can be said with confidence is narrower. English mouth is native Germanic. Its closest relatives still mean mouth. A western Indo-European word for chin, jaw, or mouth is the usual deeper comparison. The precise verbal root, if any, is not settled.
From opening to speech, river, and mood
Old English already used mūþ for more than anatomy. Late Old English extends it to the outfall of a river, the place where water leaves a channel and enters a larger body. By the thirteenth century the same image covers the opening of a bottle, a cave, or anything with capacity. Those senses are still ordinary.
Speech is never far away. A mouth is where words come from, so the noun has long stood for utterance, testimony, and a person who speaks for others — later a mouthpiece. Slang still calls a loud talker a mouth. To have mouths to feed treats people as eaters rather than speakers. Idioms pile up around the same image. Word of mouth for oral as against written news is on record from the 1550s. To put words in someone’s mouth, meaning to ascribe speech they did not make, appears in the late fourteenth century. Down in the mouth, dejected, is a seventeenth-century picture of a face with the corners turned down.
The verb followed the noun. Early uses mean simply to speak. In the seventeenth and eighteenth centuries mouthing often meant speaking in a pompous or affected way. The quieter sense, forming words with the lips without sound, is modern, dated to the mid-twentieth century.
A missing letter that tells the story
The most concrete fossil in the modern spelling is the one that is not there. German Mund still carries the n that Proto-Germanic *munþaz had. English mouth does not, because an early North Sea sound law ate the nasal and lengthened the vowel, and the later vowel shift turned that long u into the diphthong we hear now. If the deeper comparison with Latin mentum is right, English kept the opening and Latin kept the chin. Either way, the word on the page is a native survivor, not a borrowed label.