An expression meaning ‘I seek forgiveness from Allah’. 

The word Astaghfirullāh is a portion of the phrase ‘Astaghfirullāha rabbī wa atūbu ilayh’ which is recited between the two sujood of the ṣalāh (ritual prayer). The phrase’s full meaning is: ‘I seek forgiveness of Allah, My Lord and Sustainer, and I turn back to Him in repentance.’

Astaghfirullāh is also used in daily conversation as an exclamation of disapproval.