That's educational, surprised I didn't know that one.

"in mass" seems to still make sense though in plain english

What is funny is in the legal dictionary "en masse" is described as "in a mass"

http://thelawdictionary.org/en-masse/

Is "in mass" completely incorrect? I know "en masse" is the French version and what is usually said (and what I typically say/write, too), but "in mass" sounds syntactically and semantically correct to me.