Agile is a synonym of fast. Athletic is a synonym of fast.
Act means to play a part. It begins with the letter a.
Abhorrent is a synonym for horrible. It begins with the letter A.
The speedometer shows how fast a vehicle is travelling. It begins with the letter s.
Likely candidate words are: self-made. sure-fire.
"Brza hrana" is a Croatian equivalent of "fast food."Specifically, the noun "hrana" means "food." The adjective "brza" means "fast, quick, rapid." But the English loan words "fast food" also will be recognized and understood.
Five letters, but two words: at war.
CelerityAccelerateDecelerate
"Diagonal" begins with the prefix dia.
An Arabic word that begins with the letter R is ramadi which means gray in English.
Velocity
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Ambulance is a means of transportation. It begins with the letter A.