Synonyms for fast

modified

Rapid
swift, fleet, quick, speedy, brisk, flying, expeditious, express, accelerated, hasty, nimble, winged, mercurial, lightninglike, flashing, swift-footed, hypersonic, high-speed, active, electric, agile, ready, dashing, swift as an arrow, quick as lightning, like a flash, quick as thought, racing, fleeting, up-tempo, hurried, precipitate, breakneck, headlong, like a bat out of hell*, lickety-split*, like a house afire*, hellbent*, on the double*, at warp speed*.
Antonyms slow*, sluggish, tardy.
Firmly fixed
secure, attached, immovable; see firm 1.
Promiscuous
wanton, loose, wild, flirtatious; see lewd 2.
Permanent in color
fadeproof, colorfast, durable, lasting, washable, vat-dyed, indelible, waterproof, fade-resistant.

play fast and loose (with)*

behave recklessly, run wild, be careless; see deceive, misbehave.

noun

abstinence, fast day, Lent, Ramadan, Yom Kippur, banyan day, xerophagy, anorexia, hunger strike.

fast and rapid are generally interchangeable in expressing the idea of a relatively high rate of movement or action, but fast more often refers to the person or thing that moves or acts, and rapid to the action a fast typist, rapid transcription; swift implies great rapidity, but in addition often connotes smooth, easy movement; fleet suggests a nimbleness or lightness in that which moves swiftly; quick implies promptness of action, or occurrence in a brief amount of time, rather than velocity a quick reply; speedy intensifies the idea of quickness, but may also connote high velocity a speedy recovery, a speedy flight; hasty suggests hurried action and may connote carelessness, rashness, or impatience

verb

abstain from food, forbear eating, not eat, go hungry, starve, observe a fast, diet.

See fast in American Heritage Dictionary 4 Synonyms

fast ¹rapid swift fleet ²speedy quick hasty expeditious

These adjectives refer to something marked by great speed. Fast and rapid are often used interchangeably, though fast is more often applied to the person or thing in motion, and rapid, to the activity or movement involved: a fast runner; rapid strides.
Swift suggests smoothness and sureness of movement (a swift current), and fleet, lightness of movement (The cheetah is the fleetest of animals). Speedy refers to velocity (a speedy train) or to promptness or hurry (a speedy resolution to the problem). Quick most often applies to what takes little time or to what is prompt: a quick snack; your quick reaction.
Hasty implies hurried action (a hasty visit) and often a lack of care or thought (regretted the hasty decision). Expeditious suggests rapid efficiency: sent the package by the most expeditious means.