The metaphor fancy-dress ball may seem far-fetched to skeptical minds, and yet it aptly reflects a very important feature of the linguistic phenomenon under discussion: most participants of the carnival, if we accept the metaphor, wear masks, are disguised as something or somebody else, or, dropping metaphors, word-groups known as phraseological units or idioms are characterised by a double sense: the current meanings of constituent words build up a certain picture, but the actual meaning of the whole unit has little or nothing to do with that picture, in itself creating an entirely new image.
So, a dark horse mentioned above is actually not a horse but a person about whom no one knows anything definite, and so one is not sure what can be expected from him. The imagery of a bull in a china shop lies very much on the surface: the idiom describes a clumsy person (cf. with the R. слон в посудной лавке). A white elephant, however, is not even a person but a valuable object which involves great expense or trouble for its owner, out of all proportion to its usefulness or value, and which is also difficult to dispose of. The green-eyed monster is jealousy, the image being drawn from Othello1 . To let the cat out of the bag has actually nothing to do with cats, but means simply "to let some secret become known". In to bark up the wrong tree (Amer.), the current meanings of the constituents create a vivid and amusing picture of a foolish dog sitting under a tree and barking at it while the cat or the squirrel has long since escaped. But the actual meaning of the idiom is "to follow a false scent; to look for somebody or something in a wrong place; to expect from somebody what he is unlikely to do". The idiom is not infrequently used
1 O, beware, my lord, of jealousy; It is the green-eyed monster, which doth mock The meat it feeds on ...
(lago's words from Act III, Sc. 3)