لَوْ أَرَدْنَا أَن نَّتَّخِذَ لَهْوًا لَّاتَّخَذْنَاهُ مِن لَّدُنَّا إِن كُنَّا فَاعِلِينَ (Had We intended to have a pastime, We would have had it from Our own, if We were to do so. - 21:17) It means that if We were to create something for amusement, there was no need to create the earth and the sky, and We could have done so from the things around Us.
In Arabic language the word لَو is used for imaginary and non-existent things. Here also it is used in the same sense i.e. those foolish people who regard all the wonderful things of the earth and the sky objects of fun and frolic do not realize that creation on such a gigantic scale cannot be undertaken for amusement. The suggestion here is that even a person of ordinary common sense would not indulge in a project which has no serious purpose, to say nothing of Allah Ta` ala who is Supreme and Exalted.
The commonly accepted meaning of لَھو is a purposeless pastime, and this meaning has been adopted in translating this verse. According to some commentators this word لَھو is also used sometimes for wife or children in which case the meaning of the verse would be a denial of the belief of the Jews and Christians, who thought that Sayyidna ` Uzair and Sayyidna Masih (علیہما السلام) were God's sons. If Allah wished to have children why should they be from humans and not from other creatures around Him. (Allah knows best).