I have seen people(including my Guru ji)  saying शश्छोऽटि (rule from Ashtadhyayi)  as an expression shown to indicate, "not so great" "a very small thing" or "idiot"  "stupid"  etc. 

Is there any incident , story, joke, dantakathaa behind this?? 

Please can anyone throw some light on this matter.

Thanks 

KP