From Middle English fast, from Old English fæst, from Proto-Germanic *fastaz, *fastijaz, *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.
From Middle English fasten, from Old English fæstan (verb), from Proto-Germanic *fastijaną, derived from *fastuz, and thereby related to Etymology 1. Cognate with Dutch vasten, German fasten, Old Norse fasta, Gothic 𐍆𐌰𐍃𐍄𐌰𐌽, Russian пост. The noun is probably from Old Norse fasta.
Modern English dictionary
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