Onesies.
It was clearly something that had to do with 'one' but Dongwoo could never relate it to a threesome. He tried searching for it on the Internet, but it seemed like it was a name they gave to an action that only they knew.
"Don't worry, Hyung. Compared to putting it at once, this is much safer." Seojin said, brushing his hair back with an assuring smile. "And who knows, we might even get to put it together at the end. You may not even feel it."
'It's making me even more curious.' Dongwoo wondered and decided to ask. "What... What is this about?"
Both went silent, realizing they had never really explained it to him before.
"Well, Hyung, you see, when we have sex, instead of the other waiting for their turn or using your other hole, your mouth, we could..."
"No, wait." Seojun suddenly stopped Seojin from explaining further.
"What?"
"Instead of explaining, why not just demonstrate?" He asked.