"Corn sweat" is a colloquial term for evapotranspiration from cornfields, referring to the process where corn plants release water vapor into the atmosphere through their leaves. This occurs naturally as plants take up water from the soil and release it through stomata, the tiny pores on their leaves. While a natural process, the sheer scale of cornfields in regions like the Corn Belt means this process can contribute to increased humidity and make hot days feel even muggier.
So yeah, it's real.
Other sources of corn sweat may include: