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"fast" is also used as a crossword clue. If that's what you saw, the answer is often swift (5 letters).
Definitions
quickly or rapidly (often used as a combining form)
Word origin
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.
Used in a sentence
“That rope is dangerously loose. Make it fast!”
“out-lawes […] lurking in woods and fast places”
“I still hear you sayin', "Dear one, hold me fast"”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer9 curated clues
01“Moving at high speed”4 letters
02“Not slow”4 letters
03“Observing a food ban”4 letters
04“Quick in motion”4 letters
05“Quick to move”4 letters
06“Quick-moving”4 letters
07“Rapid”4 letters
08“Speedy”4 letters
09“Speedy or abstaining”4 letters
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