Discovered used to mean uncovered.
Discovered check and discovered attack

Exactly - when you find something out you have, figuratively speaking, "uncovered" it, and that's why discover/discovery has come to have its modern meaning.
Discover is a perfect name for it. It is the opposite of "cover" I think, so it is a perfect name. A word can have several meanings.

when you uncover something you know it's there before you uncover it.
when you discover something you don't know it's there until you discover it.
the word discover has a connotation of discovery - of finding something new.

when you uncover something you know it's there before you uncover it.
when you discover something you don't know it's there until you discover it.
the word discover has a connotation of discovery - of finding something new.
It does now. It didn't always. "Dis-" is just a negation as "un-" is, and any differences in shades of meaning are the ones that have grown up over the years; there was once a time when to "dis-cover" meant nothing more or less than to "un-cover".
English changes in meaning over time. The Book of Common Prayer once included a prayer that judges and magistrates should "indifferently minister justice", which in the 17th century meant they should do their job fairly, not letting whether the man before them was rich or poor affect their judgment. Today you don't pray for anyone to do an "indifferent" job because the modern meaning would have it that they were doing a mediocre job, neither good nor bad. Similarly, a couple of centuries ago when a boss prevented his staff's arrival at the office, it just meant that he got there first (Latin, praevenire).
While today "the word discover has a connotation of discovery" ([Nicholas Cage] You don't say![/NC]), it didn't at the time "discovered check" became current, and English-speaking chessplayers as a bunch are happy with the phrase as it is.
TLDR: Find something better to do than criticize idioms in someone else's language.

Discover is a perfect name for it. It is the opposite of "cover" I think, so it is a perfect name. A word can have several meanings.
- Discover is "dis" cover. Similar to "dat" cover (the other cover).
- It is possible to pair them. In some cases you can cover dis piece and cover dat piece at the same time.
- Recover is where you make a discovered attack, returning your piece back to the original position later.
- Undercover is when you play the game in stealth mode, usually while wearing a disguise.
- Cover charge is when you protect one piece while attacking ("charging" at) another.

gil-gandel, brilliant history lecture
thank you
yes, of course you are right that language is not static but an ever changing thing.

thanks to gil-gandel i now understand that "discovered attack" is chess specific terminology that was coined at a time when it also made good english sense and it has stuck with chess while the connotation of the word discover has changed; but that in itself is no reason for chess players to change their technical jargon. thus enlightened, i would like to withdraw my objection. as we say here in asia, solee, so solee.

Or maybe what I called it ''unplanned'' attack
Speak for yourself - when I make a discovered attack there's nothing unplanned about it.

" discovered check " may just be a rough tranlation of the french " echec a la decouverte " ... In the french word, the meaning is closer to the word "uncovered check" ( the words a la having no way to be translated in English in the same way that in checkmate, also possible translation of the french "echec et mat" in which the word "et" was not translated either....). Just a suggestion.

ubermensch, it's you again, what you way makes really superb sense and sounds very logical. thank you for your input. i used to speak french (forgot) but my understanding is that decouverte has the same connotation as exploration and discovery in modern french. so i think we are back to the gil-gandel theory - that the connotation of these words have changed over time. i have not gone through the history of english to confirm gil-gandel's statement but it seems very plausible because as gil points out language is a dynamic thing.

Concise Oxford English Dictionary 6th edition (1976):
discover v.t. 1. Expose to view, reveal; make known; exhibit, manifest; (arch.) disclose, betray; (Chess) give (check) by removing one's own obstructing man. 2. Find out (fact etc., that etc., unknown country); become aware that. <SNIP> [ME, f OF descovrir f. LL DIScooperire COVER 1)
So, Jamalov, in 1976 and according to the OED, your understanding of discover was only the 2nd sense; the first sense was still to expose to view. The special use in chess terminology is correctly given as a part of sense 1, not sense 2. The etimology (for those not familiar with the abbreviations) is that "Discover" comes from Middle English and before that from the Old French verb "descovrir" which in turn came from the latin "cooperire", meaning to cover, plus the prefix "DES".
My Collins dictionary from 1986 does not give sense 1 at all. It agrees with you that "discover" means "to be the first to find or find out about."
Chess terminology refers to an an attack formed by moving an intervening piece and exposing the attacked piece to the attacker as a "discovered attack" and if the attack delivers check it is called a "discovered check". This terminology is flawed.
Discovery involves exploration and an element of surprise or perhaps an element of not knowing something until the moment when the discovery is made. These aspects of discovery are absent in chess moves that are referred to as a discovered attack or discovered check.
It would be more accurate to describe these moves as uncovered attacks or perhaps exposed attacks. So for example, if we make a move such as Ne5-c6 as in the Petroff exposing the black king to check delivererd by the queen on e2, we should refer to it as an exposed check or an uncovered check.
It is not a discovered check because there is no discovery involved.