Astaghfirullah literally translates to "I seek forgiveness in Allah". In a religious way, people can recite it during meditation or say it to others to show humility- to say that Allah is greater or that goodness comes from Allah. In popular culture, people can say this if they see something wrong or shameful (context applies).
Example 1:
Person- Hatija, your lecture was truly insightful!
Hatija- Astaghfirullah, Allah gives us the knowledge.

Example 2:
A group of religious people watching a movie together when a sex scene comes on
-"Astaghfirullah" in congregation.
by skepticalcitizen February 26, 2017
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An Arabic expression of disapproval or shame, meaning "God forgive me".
They didn't put any lemon juice in the hummus. Astaghfirullah!
by LudwigVan August 23, 2004
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What Muslims say when they hear something haram and they wanna seek forgiveness from Allah
Example 1:
Raheem: I want to rail your cousin so hard before marriage
Mahmood: Astaghfirullah!

Example 2:
Eric: I ate some bacon
Mario: I ate-ah some of da salami!
Günter: Ein ate das sausages
Mustafa: Astaghfirullah!
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