When Jared Dunn brings up Meinertzhagen's haversack in season 3 episode 3 "Meinertzhagen's Haversack", he is referring to the Haversack Ruse as a necessary gambit for successfully pulling off their skunkworks project of continuing to work on the middle out compression algorithm platform while giving the appearance of working on the hardware box.
[Colonel Richard] Meinertzhagen's sources of information dwindled to the occasional prisoner caught out by patrols, and deserters. He is frequently credited with a surprise attack known as the Haversack Ruse in October 1917: during the Sinai and Palestine Campaign of the First World War, according to his diary, he let a haversack containing false British battle plans fall into Ottoman military hands, thereby bringing about the British victory in the Battle of Beersheba and Gaza.
The evidence was wrapped in paper used for sandwiches, and taken to General Kress von Kressenstein, who examined the pocket book and mused on its authenticity. According to an account by Turk Colonel Hussein Husni, Chief of Staff of 7th Army, Meinertzhagen's German-sounding name added to its genuine feel. Von Kressenstein wanted to believe it, which threw the staff into some confused arguments. The ideas of Allenby's general leave-time played on Turk preconceptions about the way British and Australians were thought to behave.
Having thoroughly devised their skunkworks, the boys are celebrating and Gilfoyle comments:
Gilfoyle: I think I might actually enjoy going to work now.
Jared: Wait, we can't enjoy going to work. Meinertzhagen's Haversack.
Jared: Meinertzhagen's Haversack. No one? Really? No one's heard of Meinertzhagen's Haversack?
Erlich: Of course I have, Jared. Just explain it to them.
Jared: Well, it's a principle of military deception. Essentially it means you have to continue to act the part. So, as far as anyone knows, we're still building a box that we hate. We need to act like it.
Essentially, the Haversack (meaning "bread bag" or "satchel" in German) ruse requires that in addition to feeding Jack Barker the false information about progress on the box, the team also has to continue acting as if they hate what they are doing and maintain the status quo of their behavior (which is also why Dinesh has to keep wearing his gold chain).
Dinesh: If we do anything differently, Barker's gonna be onto us. We have to keep complaining about Barker. We have to keep bitching about the box.
Gilfoyle: We have to keep making fun of your gold chain. We have to. We don't have any other choice.
Jared: He's right.
Erlich: It's a shame Bel Biv Dinesh, but unfortunately that chain is Poison.
Like the name "Meinertzhagen" played well to the Turks, it is also funny within the show when Jared realizes the need for the additional ruse and we hear him blurting out the Germanic phrase. From season 2, he is known to the audience to speak German in his sleep: