A sexual act. The male attaches multiple kernels of corn to a string then repeatedly shoves them up the asshole of the female. Afterwards they make popcorn with the kernels.
Guy 1: Hey man saw you with that girl last night, how'd it go?
Guy 2: Great I gave her an Iowa cornhole!!
Guy 1: Awww that's awesome, you lucky bastard
Guy 2: Yeah... want some popcorn?