peri

Almost every word in this family hides the same small Latin verb: experīrī, 'to try out, to put to the test.' It is built from per- (an old intensive, 'through, thoroughly') plus a root meaning 'to attempt' — the very same root that gives Greek peira, 'an attempt, a trial.' The whole family is a story about what happens when you try things.

Start with the most everyday member. Add the prefix ex- ('out') to experīrī and you 'try something out.' What do you have left afterward? The knowledge that trying produced — your experience. Experience is literally 'what you tried out and kept.' This is why we say knowledge is gained through experience: the word itself records the act of testing reality and walking away wiser. Strip the adjective off and you get experienced ('someone who has tried a lot') and its opposite inexperienced.

Formalize the trying and you get experiment — a deliberate, repeatable trial. A scientist does not just experience the world; she sets up a controlled try, an experiment, to see what happens. Hence experimental ('still at the trying-out stage, not yet settled') and experimentation ('the ongoing process of trying').

Now take the participle perītus, 'one who has been tested, one who has tried again and again.' Put ex- in front and Latin gives expertus → English expert: a person so thoroughly tried that the trying is over — they simply know. Expertise is the tried-and-tested knowledge such a person owns; someone who has never been tested is inexpert.

The darker branch comes from perīculum: 'a trial' in the threatening sense — the risk you run when you put yourself to the test. That word slid through Old French into English as peril, plain 'danger.' Something full of that risk is perilous, and to imperil is to push someone into it. The link is intuitive once you see it: every trial carries the chance of failure, and a serious enough chance of failure is simply danger.

The biggest surprise sits at the edge of the family — and it comes through Greek rather than Latin. Greek peira ('attempt') produced peiratēs, 'one who attempts, who tries his luck' — specifically by attacking ships. That word became Latin pirata and then English pirate. A pirate is, etymologically, just 'a try-er,' a chancer who tests his fortune by raiding. The modern 'software pirate' who copies what isn't his keeps the old flavor of someone making an unauthorized attempt.

So the pattern across the whole family is a single human truth: trying things teaches you (experience, experiment), eventually masters you in a craft (expert, expertise), but always carries danger (peril, perilous) — and the boldest try-ers of all sailed off as pirates.