Bedsheets

murder backwards

typically used as a reference to The Shining

Don't worry about it - being intelligent and assuming the best of others is nothing that you need to apologize for imho!:-)

As the other commenter pointed out, it is from that movie "The Shining". It is more than 40 years old, and based on an even older book by Stephen King. I wonder if most younger people today have even seen it, and it wasn't even all that "good", as in like production values from the modern standpoint (especially things like pacing) - though the story itself was quite excellent.

That said, it is one of those cultural, iconic "classics" like Wizard of Oz or the three stooges that most everyone seems to know about, especially in the horror genre. In particular, the main character in it starts to slowly go crazy as he sees visions of ghosts (leaving it unspecified iirc whether they are real or just a product of his delusional mindset), and one of the bits is the word "murder" spelled backwards, and written in red lettering, seemingly in blood.

Another bit is the two ghost girls that appear in the hallway - I suspect b/c it is one of the more SFW images that can be shown on the internet for this movie, but really that's all there is too it, they just stand there, creepily.

The last thing you really need to know about The Shining is the scene where the character portrayed by Jack Nicholson (edit: I misspelled his name) uses an ax to cut through the door to get to his wife, who is ofc absolutely terrified.

And there you are, you are all caught up with The Shining!:-) You are probably far more knowledgeable than I irl and in the wider sense, but I did enjoy this chance to explain what I know:-D.