Etymology
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Words related to thumb

*teue- 
*teuə-, also *teu-, Proto-Indo-European root meaning "to swell."

It forms all or part of: butter; contumely; creosote; intumescence; intumescent; protuberance; protuberant; psychosomatic; somato-; -some (3) "body, the body;" soteriology; Tartuffe; thigh; thimble; thousand; thole (n.); thumb; tumescent; tumid; tumor; truffle; tuber; tuberculosis; tumult; tyrosine.

It is the hypothetical source of/evidence for its existence is provided by: Avestan tuma "fat;" Greek tylos "callus, lump;" Latin tumere "to swell," tumidus "swollen," tumor "a swelling;" Lithuanian tukti "to become fat;" Lithuanian taukas, Old Church Slavonic tuku, Russian tuku "fat of animals;" Old Irish ton "rump."
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dumb (adj.)

Old English dumb, of persons, "mute, silent, refraining from speaking or unable to speak," from Proto-Germanic *dumbaz "dumb, dull," which is perhaps from PIE *dheubh- "confusion, stupefaction, dizziness," from root *dheu- (1) "dust, mist, vapor, smoke," also expressing related notions of "defective perception or wits." The -b has probably been silent since 13c. Related: Dumbly; dumber; dumbest. Of animals, "lacking in speech," hence "without intellect" (c. 1200).

The fork in meaning probably comes via the notion of  "not responding through ignorance or incomprehension." The Old English, Old Saxon (dumb), Gothic (dumbs), and Old Norse (dumbr) forms of the word meant only "mute, speechless;" in Old High German (thumb) it meant both this and "stupid," and in Modern German this latter became the only sense (the sense of "mute, speechless" being expressed by stumm). Meaning "foolish, ignorant" was occasional in Middle English, but the modern use in this sense (since 1823) seems to be from influence of German dumm, especially in Pennsylvania German.

dumb-cake ..., n. A cake made in silence on St Mark's Eve, with numerous ceremonies, by maids, to discover their future husbands. [Century Dictionary]

Applied to silent contrivances, hence dumb-waiter. Dumb ox "stupid man" is by 1756; dumb-bunny "stupid person" is college slang from 1922; dumb blonde "woman seen as incapable of comprehending anything complicated" is by 1936.

limb (n.1)

"part or member," Old English lim "limb of the body; any part of an animal body, distinct from the head and trunk;" main branch of a tree," from Proto-Germanic *limu- (source also of Old Norse limr "limb," lim "small branch of a tree"), a variant of *lithu- (source of Old English liþ, Old Frisian lith, Old Norse liðr, Gothic liþus "a limb;" and with prefix ga-, source of German Glied "limb, member").

The unetymological -b began to appear late 1500s for no etymological reason (perhaps by influence of limb (n.2)). The Old English plural was often limu; limen and other plural forms in -n lasted into Middle English. Since c. 1400 especially of a leg; in Victorian English this usage was somewhat euphemistic, "out of affected or prudish unwillingness to use the word leg" [Century Dictionary]. However in Old and Middle English, and until lately in dialects, it could mean "any visible body part":

The lymmes of generacion were shewed manyfestly. [Caxton, "The subtyl historyes and fables of Esope, Auyan, Alfonce, and Poge," 1484]

Hence, limb-lifter "fornicator" (1570s). Limb of the law was 18c. derisive slang for a lawyer or police officer. To go out on a limb in figurative sense "enter a risky situation" is from 1897. Alliterative life and limb in reference to the body inclusively is from c. 1200. Obsolete limb-meal (adv.) "limb-from-limb, piecemeal" is from late Old English lim-mælum.

thimble (n.)
Old English þymel "sheath or covering for the thumb," from thuma (see thumb (n.)) + instrumental suffix -el (1), used in forming names of tools (compare handle (n.)). The unetymological -b- appears mid-15c. (compare humble, nimble, etc.). Originally of leather, metal ones came into use 17c. Related: Thimbleful. Thimblerig, con game played with three thimbles and a pea or button, is attested from 1825 by this name, though references to thimble cheats, probably the same swindle, date back to 1716 (see rig (v.)).
thumbnail (n.)
c. 1600, from thumb (n.) + nail (n.). Meaning "drawing or sketch of a small size" (though usually not literally the size of a thumbnail) is from 1852.
thumbtack (n.)
tack with a broad, flat head which may be driven by pressure from the thumb, 1884, from thumb (n.) + tack (n.1).