Why does Jacob indicate "Expeditionary Forces" by performing a "digging" gesture with his hands?

Question

Jacob: Yeah. I was part of the expeditionary forces there.

Why does Jacob indicate "Expeditionary Forces" by performing a "digging" gesture with his hands?

Answer 1

According to Jacob's bio he fought in the First World War. As an American it is near 100% certain that he was a part of the American Expeditionary Forces in France. Given the nature of the fighting in that war, this would have involved a lot of digging trenches and earthworks. Hence the "digging" gesture.

The AEF helped the French Army on the Western Front during the Aisne Offensive (at the Battle of Château-Thierry and Battle of Belleau Wood) in the summer of 1918, and fought its major actions in the Battle of Saint-Mihiel and the Meuse-Argonne Offensive in the latter part of 1918.

Answer 2

It's a bad joke.

SCENE 9

INT. BINGLEY'S OFFICE-MOMENTS LATER-DAY

Jacob is facing the imposing and impeccably suited Mr. Bingley. Bingley is examining Jacob's business proposal for a bakery.

An uncomfortable silence. The sound of a ticking clock and Bingley murmuring.

Jacob looks down at his pocket--the egg has started to vibrate.

BINGLEY

You are currently working... in a canning factory?

JACOB

That's the best I can do-I only got back in '24.

BINGLEY

Got back?

JACOB

From Europe, sir. Yeah-I was part of the Expeditionary Forces there-

Jacob is clearly nervous, miming a digging action to the words "Expeditionary Forces," in the vain hope that a joke might help his cause.

_{(Fantastic Beasts and Where to Find Them: The Original Screenplay)}