"Tool On Deck" is an urban phrase used when you want to let people know you have gun. The word "tool" refers to the gun and the word "deck" refers to having your gun at the ready. The phrase was originated in the south in places like Miami & New Orleans. Was made popular by rapper Lil' Wayne on the song "9mm" on David Banners album "The Greatest Story Ever Told".
What? You wanna start some shit? TOOL ON DECK!
Don't worry, if they start something I got my "tool on deck".