The word(s) "frail" appears 2 time(s) in 2 verse(s) in Quran in Muhammad Pickthal translation.

(1) The likeness of those who choose other patrons than Allah is as the likeness of the spider when she taketh unto herself a house, and lo! the frailest of all houses is the spider's house, if they but knew.

(سورة العنكبوت, Al-Ankaboot, Chapter #29, Verse #41)
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(2) And the heaven will split asunder, for that day it will be frail.

(سورة الحاقة, Al-Haaqqa, Chapter #69, Verse #16)
(Arabic, Transliteration, Urdu (Ahmed Ali), Urdu (Jalandhry), Farsi, Yusuf Ali, Shakir, Picthal, Mohsin Khan, French, Spanish, Indonesian, Malay, German, Bosnian, Russian, Dutch, Italian, Portuguese, Tafsir - Jalal ad-Din, Tafsir - King Fahad Quran Complex)