This is a close relative of the palindrome, a string of letters that reads the same backwards as forwards (“Madam, I’m Adam”; “A man, a plan, a canal: Panama!”; “Was it a car or a cat I saw?”).
In a semordnilap the text is likewise reversed but it must turn into something different. For example, if you reverse “diaper” you get “repaid”, and if you invert “desserts” the word “stressed” appears. A more complicated example is “deliver no evil”, but you can probably invent better ones for yourself.
As semordnilap is palindromes written backwards, it’s a self-referential word, one that encapsulates within itself the thing it represents. You could hardly say that it’s common, but many earnest palindromists have accidentally discovered it, and it has some small circulation among word wizards and elsewhere.
Derrida particularly favors the figure of a “headstrong dog,” possibly because dog, a semordnilap for god, helps him to configure an immanent versus transcendent ontology.
Animal Capital: Rendering Life in Biopolitical Times, by Nicole Shukin, 2009.