Origins
The English adjective 'fast' presents one of the most instructive cases of semantic evolution in the language. Its modern primary meaning — 'moving at high speed, quick' — is not its original meaning at all. Old English 'fæst' meant 'firmly fixed, steadfast, secure, enclosed, strong,' from Proto-Germanic *fastuz (firm, fixed, solid). The sense of quickness developed only during the Middle English period, and the earlier meaning of firmness has not disappeared — it coexists with the newer sense, creating a word that appears to contradict itself. 'Hold fast' means hold firmly (the old sense), while 'run fast' means run quickly (the new sense).
The Proto-Germanic root *fastuz is well attested across the family: German 'fest' (firm, solid, fixed), Dutch 'vast' (firm, fixed), Swedish 'fast' (firm, fixed), Danish 'fast' (firm, permanent), Norwegian 'fast' (firm, fixed), and Icelandic 'fastur' (firm, fixed). In every one of these languages except English, the primary meaning remains 'firm' or 'fixed,' not 'quick.' English alone developed the speed sense, making this a characteristically English semantic innovation.
The development from 'firm' to 'quick' is usually explained through intermediate senses. One pathway runs through the idea of 'keeping close, staying firm in pursuit' — a hunter who runs 'fast' after prey is running closely, firmly, without letting go, and the notion of close pursuit naturally implies speed. Another pathway runs through the adverbial sense 'firmly, vigorously' — to run fast originally meant to run vigorously, with energy and force, and vigorous running is fast running. Both explanations are plausible, and the shift may have occurred through multiple reinforcing channels.
Old English Period
The compound 'steadfast' (from Old English 'stedefæst,' literally 'standing-firm') preserves the original meaning unambiguously. A steadfast person is one who does not waver, who remains firmly in place. 'Fastness' in its older sense means a stronghold or fortress — a place that holds firm against attack. The nautical command 'make fast' means to secure a line, to tie it firmly. These surviving uses keep the original meaning alive in Modern English alongside the newer speed sense.
The verb 'to fast' (to abstain from food) is the same word, developing from the sense of holding firmly to a rule of discipline. A person who fasts holds fast to the practice of not eating. This meaning is attested from Old English and is shared across the Germanic languages: German 'fasten' (to fast), Old Norse 'fasta' (to fast). The noun 'breakfast' is literally 'the breaking of the fast' — the meal that ends the overnight period of not eating. The compound appears in Middle English as 'brekefast' and became standard during the fifteenth century.
The verb 'fasten' (to attach firmly, to secure) is derived from 'fast' with the causative suffix '-en.' To fasten something is to make it fast — that is, to make it firmly fixed. 'Unfasten' reverses the process. A 'fastener' is a device that holds things firmly together. These derivatives all preserve the original 'firm, fixed' meaning of 'fast.'
Development
The phrase 'fast and loose' (behaving irresponsibly, being unreliable) refers to a medieval cheating game in which a belt or strap appeared to be fastened ('fast') but was actually free ('loose'). The mark was invited to bet on whether the belt was fixed or free, and the operator could make it appear either way. To play 'fast and loose' thus meant to cheat by making something seem firm when it was actually movable.
'Colourfast' (of dye that does not run or fade) uses 'fast' in its firm/fixed sense — the colour holds fast, it does not move. This compound appeared in the nineteenth century as industrial dyeing required vocabulary to describe the permanence of colours.
The adverb 'fast' can modify verbs in both its senses: 'hold fast' (firmly), 'run fast' (quickly). It is one of the 'flat adverbs' in English — adverbs that lack the '-ly' suffix. 'Fast' as an adverb of speed coexists with 'fast' as an adverb of firmness, and context alone determines which is meant. The form 'fastly' exists in some historical texts but has never been standard and is considered incorrect in Modern English.
Figurative Development
In modern informal English, 'fast' has acquired extended meanings: a 'fast' person lives recklessly or is sexually promiscuous (attested from the eighteenth century). 'Fast talk' is persuasive but deceptive speech. The 'fast lane' is the overtaking lane on a motorway, and metaphorically, a lifestyle of high speed and high risk. 'Fast food' (coined in the 1950s) describes food prepared and served quickly.
The phonological history of 'fast' is relatively stable. Old English 'fæst' had a short 'a' vowel before the consonant cluster '-st,' which lengthened in some dialects (producing /fɑːst/ in Southern British English) and remained short in others (producing /fæst/ in many American varieties). The consonants have remained unchanged since Old English.