Old English toð (plural teð), from Proto-Germanic *tanthu- (source also of Old Saxon, Danish, Swedish, Dutch tand, Old Norse tönn, Old Frisian toth, Old High German zand, German Zahn, Gothic tunþus), from PIE root *dent- "tooth." Plural teeth is an instance of i-mutation.

The loss of -n- before spirants is regular in Old English, Old Frisian, and Old Saxon: compare goose (n.), five, mouth (n.). Also thought, from stem of think; couth from the stem of can (v.1); us from *uns.

Application to tooth-like parts of other objects (saws, combs, etc.) first recorded 1520s. Tooth and nail as weapons is from 1530s. The tooth-fairy is attested from 1964.

Proto-Indo-European root meaning "to project." 

It forms all or part of: amenable; amount; cismontane; demeanor; dismount; eminence; eminent; imminence; imminent; menace; minacious; minatory; mons; montage; montagnard; monte; mount (n.1) "hill, mountain;" mount (v.) "to get up on;" mountain; mountebank; mouth; Osmond; Piedmont; promenade; prominence; prominent; promontory; remount; surmount; ultramontane.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit manya "nape of the neck;" Latin mons "mountain," eminere "to stand out;" Old Irish muin "neck," Welsh mwnwgl "neck," mwng "mane;" Welsh mynydd "mountain." 

"abuse (someone) verbally," 1941, probably ultimately from noun phrase bad mouth (1835), in African-American vernacular, "a curse, spell," translating an idiom found in African and West Indian languages. See bad (adj.) + mouth (n.). Related: Bad-mouthed; bad-mouthing.

also bigmouth "person who talks too much," 1889, American English, from big + mouth (n.). Earlier as a type of fish and the name of a capable leader of the Oglala people in the 1860s.

also blabber-mouth, "one who talks excessively and indiscreetly," 1931, from blabber + mouth (n.).

"venomous serpent of the U.S. South," 1849, so called for the white streak along its mouth; see cotton (n.) + mouth (n.).

1884, short for large-mouthed bass (1878); see large (adj.) + mouth (n.).

also loudmouth, "loud or overly talkative person," 1872, from loud (adj.) + mouth (n.). As an adjective from 1660s; loud-mouth'd is from 1620s.

early 15c., "as much as a mouth can hold," from mouth (n.) + -ful. Meaning "a lot to say" is from 1748.

also mouth-piece, 1680s, "casting fitted on an open end of a pipe, etc.," from mouth (n.) + piece (n.1). Meaning "piece of a musical instrument that goes in the mouth" is from 1776. Sense of "one who speaks on behalf of others" is from 1805; in the specific sense of "lawyer" it is attested by 1857.