Well, I see nothing wrong with it. It is a rhetorical device known as ellipsis, which involves the removing of expected words for effect.
Why is it not only correct but actually good? Ward Farnsworth ascribes these traits to ellipsis:
- a. An ellipsis involves the audience in an utterance; the reader or listener fills in the missing language, consciously or not.
- b. Missing words sometimes are a small surprise. The result may be a moment of emphasis on whatever was omitted.
- c. The omission of words can create a sense of brevity, energy, and elegance.
- d. Often an ellipsis occurs because a later phrase borrows a word from an earlier one. The effect of this can be to tie two phrases together more snugly and strengthen the link between them.
I submit that
That looks like fun, but dangerous.
at least employs the effects described in (c) and (d) above, and probably (a) as well. And as the meaning is understood immediately by all but the slowest or most obstinate minds, there is no harm done to the sense of the communication. What else could "dangerous" possibly refer to but "That"?
Also, it's much better than filling in the missing words:
That looks like fun, but looks dangerous.
or, because now fussiness has taken over, the left brain will cry for more words to be added to nail it all down even more:
That looks like fun, but it also looks dangerous.
Neither of these improves the original. One might say the latter slowly, using extra words to make sure someone (a child perhaps) got the meaning crystal clear, but in most cases such over-emphasis would not be necessary. Making something longer does not necessarily make it better. As Pascal once wrote to a friend, "I have made this letter longer than usual because I lack the time to make it shorter."
Best Answer
In your example, which is not a relative pronoun, but a determiner, pre-modifying boy. As a determiner, it can be used with animate and inanimate nouns. The sentence is grammatical, but it’s hard to understand what it means out of context. It would be more likely to occur as ‘I don't know which boy you are meeting.’
Unlike the first sentence, the second one contains not one, but two, relative clauses, but it, too, is a most unusual sentence. Its most natural manifestation would be as ‘I don't know who the boy you are meeting is’.
The first relative clause is ‘who the boy. . . is’. The second is ‘you are meeting’. This second one omits the relative pronoun, but if you want to include it, you can. In that case it would be either that, who or whom. A relative clause can be introduced by that when, as here, it forms part of an integrated relative clause, also known as a defining relative clause or a restrictive relative clause. The choice between who and whom, in both relative clauses, depends on the formality of the context, with who being less formal, and probably more frequent, but both are found in Standard English. In relative clauses, which acts differently from when it is a determiner. In a relative clause, it can only refer to inanimate antecedents.