Measure twice, cut once.

Never bring a gun to a blueprint fight.

A Pencil Pusher is a slang term for an office worker.
The description is a reference to the phrase "Measure twice, cut once," a term often used in carpentry.

http://wiki.teamfortress.com/wiki/Pencil_Pusher

The blueprints show mechanical diagrams for a level 1 Sentry Gun. The textures used for the blueprints were originally direct copies of the in-game blueprints, but were later changed to custom team-colored blueprints.[1]
The item's description is based off of the famous saying "Never bring a knife to a gunfight".
When the RED Engineer appears on the title screen with this item and the Pencil Pusher, the item appears blue.

http://wiki.teamfortress.com/wiki/Builder's_Blueprints