Whatever it is, stick to it.
Dear Word Detective: How did “fast” come to describe something either moving quickly or fixed in place, not moving at all? And does either meaning have anything to do with not eating food? — Phil Fernandez.
That’s a darn good question. There are quite a few English words that have multiple meanings, some of which appear to be the opposite of others. There’s even a term for such critters — “auto-antonyms,” also called “contranyms” or “Janus words” (after the Roman god with two faces). Some well-known examples of such words, such as “cleave” meaning “to cut apart” and “cleave” meaning “to stick to,” are really completely different words that only happen to be spelled the same way. Others (such as “bolt,” meaning “to fasten securely,” and “bolt,” meaning “to run away quick”) are actually the same word, from the same roots, usually with a fairly convoluted tale to tell.
“Fast” is just such a word, and all the senses you mention, two adjectives and a verb, sprang from a single root, the ancient Germanic root “fastuz,” which carried the general sense of “firm.” What strikes me as surprising is that “fast” hasn’t really wandered very far from its roots, and all those senses, even “fixed in place” and “moving quickly,” involve that original “firm” sense. Incidentally, that Germanic “fastuz” also gave us our modern English verb “to fasten.”
The earliest descendant of “fastuz” in English was, not surprisingly, “fast” as an adjective meaning “firmly or strongly fixed,” which first appeared in Old English. We still use this sense, of course, when we speak of “making a line fast” by tying it securely or being “fast asleep” when we snooze so deeply that we can’t be budged. A fabric that is advertised as “color fast” is promising not to fade, at least anytime soon, and when you run up against a “hard and fast” rule, don’t expect any leniency.
The development of “fast” to mean “moving rapidly” actually began with the adverbial form of “fast,” which meant “steadily, strongly, diligently.” Although these senses grew out of the idea of something being immobile, beginning in the 13th century “fast” was applied to motion or activity (such as running) that was done in a strong, determined manner. Thus “to ride fast,” for example, echoed the “get it done” sense of “to ride hard.” The use of “fast” to mean “dissipated and dissolute” (“fast living,” etc.) arose in the early 18th century and refers to using up one’s resources too quickly.
“Fast” as a verb meaning “to abstain partially or entirely from food” is almost as old as “fast” the adjective, appearing in the 8th century. Here again the underlying sense is “firmly fixed,” but in this case the attachment was originally to a religious tradition or practice (such as fasting during Lent in the Christian church). A person “fasting” is “holding fast” to a commitment, whether religious, political, or dietary.
Lastly, the phrase “to play fast and loose,” today meaning “to be careless and irresponsible” is more than just a combination of “fast” and “loose.” In England, beginning in the 16th century, there was a con run by criminals on the streets called “fast and loose,” described by a 19th century source as “a cheating game played with a stick and a belt or string, so arranged that a spectator would think he could make the latter fast by placing a stick through its intricate folds, whereas the operator could detach it at once.” Apparently this game was so widely known to be rigged that only a fool would play, and “to play fast and loose” in the modern figurative sense of “to be foolish and irresponsible” appeared almost as soon as the game itself did.