While a lot of people say the name is dumb it is a referance both litterally and figuratively to Heckler & Koch (aka HK, real manufacturers of the G3, MP5 and the other equivalent Lachmann guns). "Lachmann" is a real German surname and roughly translates to "laugher".

After completing multiple matches with the Lachmann-762 and the Lachmann-556, the Lachmann Sub MP5 will be unlocked once the Lachmann-556 hits Level 12 in Modern Warfare 2. It should be mentioned that an excellent build has sourced the Attachment setup for Lachmann-762 above from The Loadout.