The Type 89 grenade discharger, commonly known as the "knee mortar" was a Japanese light infantry weapon introduced in 1929 which blurs the lines between grenade launcher and mortar. Like a mortar, it fires propelled explosive bombs in a high-angle indirect fire role, but it has a rifled barrel and uses a range adjustment mechanism very different from most mortars.
The knee mortar proved to be a very capable and effective weapon in WWII against US forces. It was accurate, effective, and perhaps most importantly, light and very fast to put into action. The closest comparable US weapon was the 60mm light mortar, which had a more effective projectile but was significantly slower to use.
Of course, the "knee mortar" nickname was based on the theoretical belief that one was supposed to rest the curved baseplate of the weapon on a leg while firing, which would actually have resulted in a broken leg. The baseplate was curved to allow it to dig into soft soil and be used against objects like logs and roots.