fess

The root fess comes from Latin fatērī, "to acknowledge, to admit" — with its past participle stem fass-/fess-. It is itself a frequentative of fārī, "to speak," so at its heart fess is about putting something into words out loud. What makes the family interesting is that the same act — saying something openly — splits into two very different emotional worlds depending on the prefix.

Add con- (intensive, "fully, thoroughly") and you get confess (con + fatērī): to say something completely, to hold nothing back. The thing you say out loud is usually a fault or a secret, so confessing carries the weight of guilt and relief. From it grow confession (the thing confessed), confessor (the priest who hears it, or the one who confesses), and confessional (the booth where it happens). The whole con- branch is private, intimate, often religious — speaking into someone's ear.

Add pro- ("forward, before others, in public") and the act turns outward. Profess (pro + fatērī) is to declare something in front of people — to state your belief or claim openly. Here is the surprise the whole family pivots on: in the Middle Ages, to "profess" meant to take the public vows of a religious order — you stood before the community and professed your faith. The noun profession first meant exactly that solemn public declaration. Over centuries the religious sense faded, but the idea of a publicly declared, committed calling survived — and that is precisely how "profession" came to mean a vocation, an occupation like law or medicine. A profession is, literally, the work you have publicly declared yourself bound to.

From the same pro- branch comes professor: in medieval universities, a professor was someone who publicly professed (taught and declared) a body of knowledge — one who openly expounds a subject. And professional is simply the adjective of profession: belonging to a declared, paid calling — which is why "professional" now means skilled, expert, doing it for a living as opposed to amateur.

The pattern: fess always means "speak it out loud." con- turns that inward and private (admitting a fault); pro- turns it outward and public (declaring a belief, then a vocation, then expertise).