fast

From Middle English fast, fest, from Old English fæst (“firm, secure”), from Proto-West Germanic *fast, from Proto-Germanic *fastuz; see it for cognates and further etymology.

The development of “rapid” from an original sense of “secure” apparently happened first in the adverb and then transferred to the adjective; compare hard in expressions like “to run hard”. The original sense of “secure, firm” is now slightly archaic, but retained in the related fasten (“make secure”). Also compare close meaning change from Latin rapiō (“to snatch”) to Latin rapidus (“rapid, quick”), from Irish sciob (“to snatch”) to Irish sciobtha (“quick”).

From Middle English fast, faste, from Old English fæste, from fæst + -e (adverb-forming suffix).

From Middle English fasten, from Old English fæstan (“to fast”), from Proto-West Germanic *fastēn, from

Proto-Germanic *fastāną (“to fast”), from the same root as Proto-Germanic *fastijaną (“to fasten”), derived from *fastuz, and thereby related to Etymology 1.

The religious sense is presumably introduced in the Gothic church, from Gothic 𐍆𐌰𐍃𐍄𐌰𐌽 (fastan, “hold fast (viz. to the rule of abstinence)”). This semantic development is unique to Gothic, the term glosses Greek νηστεύω (nēsteúō), Latin ieiuno which do not have similar connotations of "holding fast".

The feminine noun Old High German fasta likely existed in the 8th century (shift to neuter Old High German fasten from the 9th century, whence modern German Fasten).

The Old English noun originally had the sense "fortress, enclosure" and takes the religious sense only in late Old English, perhaps influenced by Old Norse fasta.

The use for reduced nutrition intake for medical reasons or for weight reduction develops by the mid-1970s, back-formed from the use of the verbal noun fasting in this sense (1960s).

From Middle English faste, fasten, from Old English fæsten (“fast, fasting”), from Proto-West Germanic *fastuni, from Proto-Germanic *fastu(b)nją (“fast, fasting”). Cognate with Old Frisian festene, feste (Modern West Frisian fêsten, fêste), Middle Dutch vastene (Modern Dutch vasten), Middle Low German vastene (Modern German Low German Fasten), German Faste, Fasten (“fast, fasting”), Gothic 𐍆𐌰𐍃𐍄𐌿𐌱𐌽𐌹 (fastubni, “fasting”). Possibly merged with Old Norse fasta (“fast, fasting”), whence also Danish faste (“fast, fasting”), Swedish fasta (“fast, fasting”).