Entries linking to small-mouth
Old English smæl "thin, slender, narrow; fine," from Proto-Germanic *smal- "small animal; small" (source also of Old Saxon, Danish, Swedish, Middle Dutch, Dutch, Old High German smal, Old Frisian smel, German schmal "narrow, slender," Gothic smalista "smallest," Old Norse smali "small cattle, sheep"), perhaps from a PIE root *(s)melo- "smaller animal" (source also of Greek melon, Old Irish mil "a small animal;" Old Church Slavonic malu "bad"). Original sense of "narrow" now almost obsolete, except in reference to waistline and intestines.
My sister ... is as white as a lilly, and as small as a wand. [Shakespeare, "Two Gentlemen of Verona," 1591]
Sense of "not large, of little size" developed in Old English. Of children, "young," from mid-13c. Meaning "inferior in degree or amount" is from late 13c. Meaning "trivial, unimportant" is from mid-14c. Sense of "having little property or trade" is from 1746. That of "characterized by littleness of mind or spirit, base, low, mean" is from 1824. As an adverb by late 14c.
Small fry, first recorded 1690s of little fish, 1885 of insignificant people. Small potatoes "no great matter, something petty or insignificant" is attested by 1924; small change "something of little value" is from 1902; small talk "chit-chat, trifling conversation" (1751) first recorded in Chesterfield's "Letters." Small world as a comment upon an unexpected meeting of acquaintances is recorded from 1895. Small-arms, indicating those capable of being carried in the hand (contrasted to ordnance) is recorded from 1710.
Old English muþ "oral opening of an animal or human; opening of anything, door, gate," from Proto-Germanic *muntha- (source also of Old Saxon, Old Frisian muth, Old Norse munnr, Danish mund, Middle Dutch mont, Dutch mond, Old High German mund, German Mund, Gothic munþs "mouth"), with characteristic loss of nasal consonant in Old English (compare tooth), probably an IE word, but the exact etymology is disputed. Perhaps from the source of Latin mentum "chin" (from PIE root *men- (2) "to project," on the notion of "projecting body part"), presuming a semantic shift from "chin" to "mouth."
In the sense of "outfall of a river" it is attested from late Old English; as the opening of anything with capacity (a bottle, cave, etc.) it is recorded from mid-13c. Mouth-organ attested from 1660s. Mouth-breather is by 1883. Mouth-to-mouth "involving contact of one person's mouth with another's" is from 1909.
Word of mouth "spoken words, oral communication" (as distinguished from written words) is by 1550s. To put words in (someone's) mouth "represent as having said what one did not say" is from late 14c.; to take the words out of (someone's) mouth "anticipate what one is about to say" is from 1520s. To be down in the mouth "dejected" (1640s) is from the notion of having the corners of the mouth turned downward.