Origins
The word 'fiber' (British spelling 'fibre') entered Middle English around 1390 from Old French 'fibre' (fiber, filament, thread), from Latin 'fibra' (a fiber, a filament, a lobe or tissue of an organ, the tip of a root). The deeper etymology of Latin 'fibra' is uncertain. Some scholars have proposed a connection to Latin 'fīlum' (thread), but the phonological correspondence is irregular and the link remains speculative. The word may derive from a pre-Indo-European Mediterranean substrate language, as several Latin words related to natural materials and agriculture do.
In classical Latin, 'fibra' had a wider semantic range than modern English 'fiber.' It referred to plant fibers (the stringy parts of roots and stems), to the internal fibrous tissues of animals (entrails, organ tissues), and to the fine threads visible in the cross-section of wood or meat. Roman augurs — the haruspices — examined the 'fibrae' of sacrificial animals' livers to read omens and predict the future. The fibers, lobes, and markings of the liver were believed to encode divine messages. This divinatory use gave 'fibra' an association with hidden inner nature — the fibers of a thing revealed its true character.
This association between fibers and essential character survived into modern English. The phrase 'moral fiber' (or 'moral fibre') — meaning inner strength, integrity, the essential quality of a person's character — emerged in the nineteenth century and draws on the old Latin intuition that the fibers within a body are its deepest truth. 'Every fiber of my being' uses the same metaphor: the fibers are the most fundamental structural elements, the threads from which the whole person is constructed.
Development
The textile sense of 'fiber' — a thin strand of natural or synthetic material used in spinning and weaving — became dominant in English during the industrial era. Cotton fiber, wool fiber, silk fiber, linen fiber — each textile material is described in terms of the individual strands from which thread and cloth are made. The word provides a level of abstraction below 'thread' (which is spun from fibers) and above the molecular level (which describes the chemical composition of the fibers themselves).
The dietary sense of 'fiber' — plant material that passes through the human digestive system largely undigested — is a twentieth-century development. Nutritionists began using 'dietary fiber' in the 1950s and 1960s to describe the cellulose, hemicellulose, pectin, and lignin found in fruits, vegetables, and grains. This sense has become one of the word's most common uses, appearing on food packaging and in nutrition guidelines worldwide.
The technological sense — 'fiber optics,' 'fiber-optic cable' — emerged in the 1960s when scientists developed glass and plastic fibers thin enough to transmit light over long distances. Fiber-optic cables now carry the vast majority of the world's telecommunications and internet traffic. The word 'fiber' thus spans from ancient Roman divination to the infrastructure of the digital age, from reading entrails to transmitting data.
Scientific Usage
The adjective 'fibrous' (containing or consisting of fibers) entered English in the seventeenth century. Medical derivatives include 'fibrosis' (excessive fibrous tissue formation, as in cystic fibrosis or pulmonary fibrosis), 'fibril' (a small fiber), 'fibrillation' (rapid irregular contraction of muscle fibers, particularly in the heart — atrial fibrillation), and 'fibroma' (a benign tumor of fibrous tissue). The medical vocabulary reflects the importance of fibrous tissue in the body's structure: muscle, connective tissue, tendons, and ligaments are all fundamentally fibrous.
The American spelling 'fiber' and the British spelling 'fibre' diverged in the nineteenth century, following the general pattern of American simplification (color/colour, center/centre). Both spellings trace to the same Old French 'fibre,' which itself reflects the Latin 'fibra.'