This may relate to an awesome secret boner from every time Anita from accounts walks past, shaking her tatas.
This may relate to your awesome secret stash of porn that you look at when you should be doing your homework.
Either way, the phrase "Under The Desk" relates to something that is undoubtedly awesome but not many people know about it so is secret.
Awesome and secret.
Yeah man, your new tailor made trainers are well under the desk!
Carla has awesome BJ technique. It's so far under the desk. Literally. Who knew?!