prob

The root prob comes from Latin probāre — "to test, to try, to prove worthy" — which itself grew out of probus, meaning "good, upright, honest." The connection is the key to the whole family: for the Romans, you found out whether something was good by testing it. A coin was probāre to see if it was genuine; a soldier was probāre to see if he was fit. Testing and judging-as-good were two sides of the same act.

This single idea then branched in several directions, depending on the prefix.

The plainest branch is prove itself: to put something to the test and so establish that it is true. Add dis- ("reverse") and you get disprove — to test something and show it is false. Add ap- (a form of ad-, "to/toward") and the "judge good" sense comes forward: approve is to test-and-find-good, hence to sanction or to like; its formal noun approbation is official, weighty praise.

A second branch turns the testing into a quality. If something can survive testing, it is probable — literally "able to be proved," which softened into "likely." From there: probably, probability (the measure of how likely), and the negative improbable (here im- genuinely means "not" — not able to be proved, unlikely).

A third branch keeps the physical act of testing alive. To probe is to test by poking, exploring, or investigating — a surgeon's probe, a space probe, a police probe. And probation is a fixed period of testing: a new employee on probation is being tried out; an offender on probation is free but under test, watched to see if they behave.

A fourth branch returns to probus, the "upright" sense, and skips testing almost entirely: probity is moral integrity — the quality of a person whose character has been proved sound.

Finally, a darker branch uses re- ("back, against"). To reprove is to test someone and judge them against a standard — to rebuke; the noun is reproof. Push that to its extreme and you get reprobate: literally "tested and rejected," once a theological term for a soul condemned, now a hardened, unrepentant scoundrel.

The pattern to remember: prob always involves putting something to the test. The prefix tells you what happens to the verdict — proven true (prove), shown false (disprove), found good (approve), judged likely (probable), or judged and rejected (reprobate).