(noun)
A secret or semi-secret plan or set of intentions created by a female human being. Said plan must directly involve her vagina in some way.
The hidden vagenda is most often encountered in the form of a female attempting to seduce someone she is romantically attracted to.
It can also refer to any type of secret plan a female or females are executing based solely on the fact that the plan is secret and involving only females.
Kayla: I need to figure out a way to get his attention.
Jessica: okay why don't you dress up all hot and I'll dress down and we could go eat at his restaurant.
Kayla: yes that's perfect! Thank u so much babes!
Jessica: yay it'll be fun! Our little hidden vagenda.
Kayla: lol weirdo