The word(s) "unthankful" appears 2 time(s) in 2 verse(s) in Quran in Mohsin Khan translation.

(1) And if We send a wind [which would spoil the green growth (tilth) brought up by the previous rain], and they see (their tilth) turn yellow - behold, they then (after their being glad,) would become unthankful (to their Lord Allah as) disbelievers. [Tafsir At-Tabari]

(2) And indeed We bestowed upon Luqman Al-Hikmah (wisdom and religious understanding) saying: "Give thanks to Allah." And whoever gives thanks, he gives thanks for (the good of) his ownself. And whoever is unthankful, then verily, Allah is All-Rich (Free of all needs), Worthy of all praise.