Sexual references.
"Shoto, what's the first word you think of when you hear the letter A?"
"Ah."
"B?"
"Bee."
"C?"
"She."
"The fuck? D?"
"Dee…d."
"Your answers hardly make any damn sense."
"What about you, Katsuki? What's the first thing you think of when you hear the letter A?"
"Ass."
"B?"
"Bitch or balls."
"C?"
"Cum."
"Do I even have to ask for D?"
"Course ya don't hafta ask for it. I'll give it whenever."
"What?"
"You oblivious fuck. Anyway, dick."
"I figured."
A/N:
there is an actual reason why todoroki's responses are what they are.