In Leo, in an entry in the thread for "Symmetrieachse":

Kann ich fünfe gerade sein lassen und in jedem Fall "symmetry axis" verwenden?

I had never seen this expression ("fünfe gerade sein lassen") before. My dictionary translates it as "turn a blind eye; look the other way", which makes sense in the context. My question is, where does the expression come from? What is the original meaning?

Thanks!