This is a subject adopted by a nerd to conceal his or her typical intellectual pursuits. The selected subject is often something not associated with nerd/geek culture but the nerdy persona can invest his or her intellectual capacity into. Instead of seeming pedantic the knowledge of the subject may be seen as admirable. The subject can be almost anything such as sports, agriculture, beer, cars, or crafts.
The interest goes beyond being a hobby as the nerd using nerd cover will either avoid or down play other subjects that might reveal their innate personality. Nerd cover can be used to redirect a conversation from traditionally nerdy subjects to being used to dilute obvious nerdiness in an individual's interests.
"Man, Brian seems like a total nerd."
"Maybe, but did you know he keeps goats and chickens?"
"That's just his nerd cover, don't get him started on the latest zombie film."
"Wow, Warren's fantasy football team is unbeatable!"
"Yeah, football is his nerd cover. He's a mathematician so the numbers come naturally to him.