Warning: some might find an inappropriate word (or two!) in this post.
A man came to visit the famous companion Huẓayfa b. al-Yamān (d. 36 AH). In those early days, some of the houses of Madinah didn't have full doors - just curtains that would be lifted if you wanted to enter. This man walked up to the curtain, lifted it up without giving any warning, peered inside, and said, "May I enter?"
Ḥuzayfa (r) was sitting right there, and angrily replied, "As for your eyes, they have already entered. It is your ass that has yet to enter!"
أما عيناك فقد دخلتا ، وأما استك فلم تدخل!
Reported by al-Bukhārī in his Adab al-Mufrad (no. 1090).
It is, of course, completely forbidden to look into a private residence without permission, and there are aḥādith about the permissibility of physically harming someone who does so! The Quran itself commands us to seek permission before we enter into someone's house. Hence, Huẓayfa's (r) reply is completely warranted: it is as if he is telling the man that asking permission *after* looking into the house makes no sense! The purpose of asking permission is to protect one's gaze from seeing what it should not see!!
And yes, the Arabic word است has pretty much the same connotations as its English equivalent, and is used in a derogatory (may I say 'cheeky'?) sense to reference the rear-end of us human beings.
On a linguistic note, and of course completely unrelated to the morals of this story, I find it intriguing that the Arabic and English words are so similar, which in this case seems to indicate a common cognate from a very ancient language. The English word for buttocks is found in Old English as ærs, which is derived from the Proto-Indo-European root ors-. And some Semitic languages have 'است' - seems almost certain that there must be a cognate that goes back many thousands of years which evolved into both Semitic and Latin/Romance languages. Words that are not cross-cultural but emanate from a common ancient language are quite rare. (Speaking of است and ancient cognates, would it be too impolite to silently mention ضراط and its obvious English equivalent - surely someone should write an academic paper on this, no? 🤨).
Anyways, enough of hindsights and rear-endings. Let's leave all that language stuff behind us, shall we?
The point of the anecdote is: Sometimes, foolish people need to be reprimanded in a severe or sarcastic manner so that they don't do the same mistake again!