This comes from the heavy cotton padded suits worn by the Red Chinese in the Korean War.
Rumor had it that the thick padding would soak up water and freeze, and the Carbine bullet would fail to penetrate.
There may be some truth to this because the Carbine was mis-used in the Korean War by being used at longer ranges.
The Carbine was intended to be a 100 yard replacement for the pistol.
When used at the longer ranges typical in the Korean War, penetration was less.
However, at closer ranges the Carbine has long been known as a good penetrator, including of bullet proof vests.