SIR is not an acronym and it does stand for anything. But I have seen a lot of people saying the word SIR is the acronym for Slave I Remain with the explanation that the word started during the reign of aBritish colonial who would want Indians to be addressing them as Sirs. This is not true because the word 'Sir' is being used in the United Kingdom untill today.
However, Sir is used as a sign of respect or honor for men. You can also use it instead of writing the person's name especially when you do not their names. This is used by young people as they are relating with their elders, and I have not seen a situation where a young person is regarded as Sir by an elderly person unless the young man is in a place of authority. The equivalent respect for women is 'Madam'. Also, the word can be used to honor people in authority.
E. Barnes, Professional Gamer, Professional Gamer, Washington
Answered May 09, 2019
There are different meanings of SLR depending on what you would use it for. A lot of people use SLR in order to tell people that they are sorry because they have responded late to the text. This means, “Sorry Late Reply.” Some people may say that this is something that is commonly used by millennials when they are chatting with their friends.
There are also some people who use the term SLR for the single-lens reflex camera. A lot of photographers still prefer using DSLR as compared to mirrorless cameras or they also know that the use of different lenses will still be vital in order to take amazing pictures.