The belief that when you are engaging in sexual intercourse with a woman who is pregnant, they can allow their semen to be inserted to the woman's vagina without fear of impregnation.
"Ever since Autumn became pregnant, Kyle has been letting it fly."
"After Katie became pregnant, Scott asked her if he could let it fly. But, she laughed at him. "
"Having a false understanding of the definition, Eric thought he had a free pass to fuck Dennis in the ass in a restroom. Now Eric has AIDS, not HIV, but full blown AIDS."