I never heard this expression until moving to Maryland . Floridians have a very limited vocabulary when it comes to describing anything of a sexual nature. Anyway, to 'knock a person down' is to seduce them. The imagery is something like finding a hottie and steamrolling them into having sex with you. Sounds a lot easier than it is, and we all know that. But you get the picture. In context, "Gawd, I'd like to knock her down!"