... como si tuviese una mazorca en el culo

As a native speaker of English with an academic working background of Spanish, I can usually tell when something has been translated from one language to the other without any cultural understanding of why or how it might or might not be understood.
I used to call the scum of the earth basura de la tierra and the name I gave Caucasians was pan blanco, meaning white bread.
Sometimes there are just expressions that are created in the moment that seem to fit well. Seemingly, there are endless possibilities to describing people's personalities using expressions of the anus. Dumb-ass, ass hole, head up one's ass, couldn't find his ass with two healthy hands and a flashlight... We used to say that someone with a sour puss often looked as if he/she/they/it were walking around with a cob up his/her/their/its ass. That one didn't always go over too well, since you could never be sure if having a cob up one's ass were a good thing or a bad one.
having a corncob up your ass According to the idiom dictionary, the corncob may be replaced with a poker. Lovely!
On a strange note, I was at a ritual this evening and was asked, "What's the oldest person you'd ever had sex with?" Hmm... not generally a question that comes to mind. I said 83. Sadly, he was in better shape than I was; must have been the vegan diet. He sure didn't have any cob up his ass!