Disclaimer - I don't own Harry Potter. This is written for the Froday Flash Fiction amnesty challenge for November 2021. The prompt I am filling is 11.07 - naughty words. as for the title, it's actually "Son of a...", however, there's a rule on this site about keeping the title G rated and Son of a Dog is the G rated way of doing "Son of a...", though Son of a Gun is another method. Son of a Dog fits better.

Son of a...

"I'm literally a son of a bitch."

Regulus looked up at his older brother from where he sat on the couch in the drawing room as Sirius smirked ear to ear at him. "That's not funny."

"Yes it is." Sirius then added, "It's an inside joke."

"What is an inside joke?"

"It's a joke you need inside information to get."

"I see."

"But you don't see because you don't have the inside information."

"Now that's funny," Regulus said, though he didn't smile or laugh.

"What?"

"What you just said. It's funny."

"It is not."

"Inside information."

"How so?"

"You made a play on my words, for starters. Not that you were smart enough to realize you'd done that."

"Hey!"

"And, well, I know something you don't know." A smirk broke across Regulus' face. A rather smug smirk that Sirius didn't like.

"And what's that?"

"Why should I tell you?"

"So I can get the joke."

"But part of what makes it funny is you not knowing."

"Reggie!"

"Let's just say you won't want maman to find out about that let alone what you just inferred her to be."

Sirius stared at him. "Wait. Do you—how?"

Regulus said nothing.

"You can't possibly be privy to the inside information for that joke!"

"I told you the joke wasn't funny, though I guess Potter and what's his name would think it funny."

"You mean Lupin."

"Lupin probably wouldn't."

"You're pulling my leg. You couldn't possibly know."

Regulus shrugged, returning to his book, the epitome of emotionless. "And you'll never know for sure what I know."

"You little brat!"

"Don't let maman here you talk like that."