I am looking for idioms or informal/slang/colloquial expression for some people that make you think that they are able of building a skyscraper, constructing a spaceship, playing the piano better than Mozart or something very fancy that requires remarkable skills, but when it comes to practice they prove to be completely inadequate.
There are tons of them. There's a whole sub-genre based roughly off the phrase "all talk (and no substance)". These generally have the form "all X, no Y".
Others culled from synonyms on the wictionary link below include:
- All bark and no bite.
- All booster, no payload.
- All bubbles, no bath.
- All crown, no filling.
- All foam, no beer.
- All ham, no let. (For you fans of The Bard)
- All hammer, no nail.
- All icing, no cake.
- All lime and salt, no tequila.
- All mouth and no trousers. (A corruption of All mouth and trousers)
- All shot, no powder.
- All show, no go.
- All sizzle and no steak.
- All talk, no walk.
- All wax and no wick. (Alternatively All wick and no wax)
My personal favorite from cattle country here in the USA is to say that a person is all hat and no cattle.
As a note to outsiders, for some people wearing a cowboy hat has become something of a cultural statement. Thus many people wear them who, if confronted with an actual cow, would have no clue which end the "moo" comes out.
I believe a good idiom that describes this situation is blowing your own trumpet.
When someone is said to be “full of hot air”, it means that he or she talks rather a lot about topics he or she doesn't really understand.
braggart would be an appropriate and pretty generic term for such a person. This word would be appropriate in a formal or informal context. (Note that the verb "to brag" is pretty much synonymous with "to boast", so this is a pretty direct noun formation.)
From the examples you give, I would probably use the adjectives delusional and arrogant (narcissistic if you want to sound pompous, or cocky in slang) to describe such a person. For the former, an appropriate idiom might be:
He/she is on crack.
What is he/she smoking?
both alluding to the use of mind-altering drugs (though not usually literally).
Because the person can't actually do what they say they can, you could say that they cannot put their money where their mouth is:
If someone puts their money where their mouth is, they back up their words with action.
I know you’re looking for a colloquial idiom, but there does actually exist a (rare) word that means this very thing: jactator. It’s a direct borrowing from the identical Latin word. The OED marks it as “obsolete rare-0”, with the last citation given from back in the 18th century.
Somewhat more current and less rare is the related term jactation, whose sense 2 is “Boasting, bragging, ostentatious display.” It is not marked as obsolete or rare, or even archaic. Here are that sense’s citations:
- 1576 J. Woolton Christian Man. sig. H.iiiiv, If wee vse them with excesse, fylthy pleasure, vaine iactation:‥we abuse Gods good gyftes.
- 1604 T. Wright Passions of Minde (new ed.) i. vi. 26, I could adde‥Envy, Emulation‥Iactation or Boasting.
- 1825 London Mag. I. 379 There is no surer sign of vulgarity than jactation of gentility.
- 1886 Saintsbury in Macmillan's Mag. July 171 The tedious burlesque, the more tedious jactation which disfigure his work.
Its pronuncation is /dʒækˈteɪʃən/ , and its entymology is:
Latin jactātiōn-em, n. of action from jactāre to throw, toss about, discuss, boast of, refl. to talk boastfully, make an ostentatious display, frequentative of jacĕre to throw; compare French jactation (Cotgrave).
I suspect that jactator has strong potential for all sorts of intentional misunderstandings in various wicked puns. :)
Probably only students of Latin or speakers of derived Romance tongues would understand its original meaning. For example, jactarse de algo is “to boast of something” in Spanish, and is a perfectly common use, not a rare one.
Google Ngrams shows scant but measurable use of jactator compared with somewhat more frequent use of jactation, which actually seems to be on the rise since 2000. It might be used in its pathology sense, though.
Scott Adams suggests "Topper".
Some people can't walk the talk.
One of my favorites: Blowhard
grandiose 1 : characterized by affectation of grandeur or splendor or by absurd exaggeration Example: They did not believe his grandiose claims.
That fellow is grandiose; he told everyone he discovered the Fountain of Youth, but it was only a beer keg. I said, "You're joking!", but he was serious.