off-chance n.

by c. 1200 as an emphatic form of Old English of (see of), employed in the adverbial use of that word. The prepositional meaning "away from" and the adjectival sense of "farther" were not firmly fixed in this variant until 17c., but once they were they left the original of with the transferred and weakened senses of the word. Meaning "not working" is from 1861.

Off the cuff "extemporaneously, without preparation" (1938) is from the notion of speaking from notes written in haste on one's shirt cuffs. In reference to clothing, off the rack (adj.) "not tailored, not made to individual requirements, ready-made" is by 1963, on the notion of buying it from the rack of a clothing store; off the record "not to be publicly disclosed" is from 1933; off the wall "crazy" is 1968, probably from the notion of a lunatic "bouncing off the walls" or else in reference to carom shots in squash, handball, etc.

c. 1300, "something that takes place, what happens, an occurrence" (good or bad, but more often bad), especially one that is unexpected, unforeseen, or beyond human control, also "one's luck, lot, or fortune," good or bad; in a positive sense "opportunity, favorable contingency;" also "contingent or unexpected event, something that may or may not come about or be realized." It is from Old French cheance "accident, chance, fortune, luck, situation, the falling of dice" (12c., Modern French chance), from Vulgar Latin *cadentia "that which falls out," a term used in dice, from neuter plural of Latin cadens, present participle of cadere "to fall" (from PIE root *kad- "to fall").

In English frequently in plural, chances. The word's notions of "opportunity" and "randomness" are as old as the record of it in English and now all but crowd out its original notion of "mere occurrence." The meaning "fortuity, absence of any cause why an event should happen or turn out as it does, variability viewed as a real agent" is from c. 1400.

Chance is equivalent to the mathematical concept of probability, which is a precisely measurable factor enabling the accurate prediction of average outcomes over long runs of random events — the longer the run, the more accurate the predictions. Luck is at best a platitude and at worst a superstition. [David Partlett, "A History of Card Games"]

Main chance "probability that offers greatest advantage," hence "thing of most importance" is from 1570s. The mathematical sense of "probability, likelihood of a certain outcome" is from 1778, hence the odds-making sense of "balanced probability of gain or loss."

To stand a chance (or not) is from 1796. To take (one's) chances "accept what happens" (early 14c.) is from the old, neutral sense; to take a chance/take chances is originally (by 1814) "participate in a raffle or lottery or game;" the extended sense of "take a risk" is by 1826.