back-pocket knowledge 

Knowledge that one knows but only brings up when the situation presents itself.

Johnny knew he wouldn't need to know that in english, "mis bolas son picazón",means "my balls are itchy", but he remembered it just for the sake of back-pocket knowledge.

back-pocket knowledge by Dat Dude 1. July 7, 2011

Backpocket knowledge 

Backpocket knowledge consists of useless facts that serve no purpose but are still interesting to know nonetheless

Group of smart people: *having an intense conversation about the universe*
Bob: Leaves are green because they have chlorophyll.
Group: Thank you bob for sharing your backpocket knowledge with us.

Backpocket knowledge by TheBigBlackOne May 20, 2017