Because they have a weird,
INAPPROPRIATE relationship.
The Big Bang Theory Season 8, Episode 8
or some INAPPROPRIATE comment?
The Big Bang Theory Season 7, Episode 12
No, I'm sorry, that was INAPPROPRIATE.
The Big Bang Theory Season 8, Episode 23
It was INAPPROPRIATE to discuss Leonard
and Penny's sex life...
The Big Bang Theory Season 3, Episode 2
Sheldon, I do hope you forgive me
for my INAPPROPRIATE behavior last night.
The Big Bang Theory Season 3, Episode 11