What is the meaning of atammayata and where is it used?

What is the meaning of the word *atammayata*? Is it Pali or Sanskrit? I couldn't find it in any Pali-English dictionary. Where is it used and in what context? Please provide references from suttas and/or commentaries. From a Google search, I found various translations like "unconcoctability" or "non-identification" and one person suggested that it's a synonym for "not-self". Is that the case? How is *atammayata* different from not-self? In this question , it was translated as "renunciation equanimity". Why is *atammayata* important? I discovered it in this answer .