Simon says

Your dictionary is right but that definition is incomplete.

First, the game referred to is a children's group game where one child gives some commands to the others, simple things such as "Sit down quickly"; "run around in a circle"; "lift your right foot" etc. etc. etc. The trick is, the children have been told to follow these directions only if the leaders says "Simon says [do this or that]." If he/she only gives the command, without saying "Simon says," the others should do nothing but wait for the proper command. If the leader of the group is clever and a quick talker, some of the kids will get confused and start carrying out orders they're not supposed to (because the leader didn't says "Simon says.")

Okay, that's the basic game. But the expression has been adapted from this game and nowadays is sometimes used in a colloquial idiomatic sense. In this usage "Simon says" means that this particular dictum is "the official version" or the commonly accepted way of doing things.

That'sa rather convoluted explanation, I know but I hope it helps to clarify matters.

A couple of things to add. When playing "Simon says," one child is "it," and they stand with their back to the others, so that they cannot see them. Therefore, if someone obeys a command which was not preceded by "Simon says," the child who is "it" can't really know except by the reaction of the other children (who usually tell on the player who has made the mistake). If someone obeys a command without having been told "Simon says," he or she becomes "it" and trades places with the former "Simon." So, in the figurative use, it also can mean obeying commands or following rules when no one is actually paying attention--no one is supervising.

(I played that game as a small child, and it is amazing how willingly other children will inform on one another.)

Thank you both.