In fact it means is they have a secret... 의해 saying that they can be trying to convince themselves that they don't have secrets 또는 it could be a facade to make sure other people won't pry 의해 asking personal questions... Either way i think if a person says that they are lying because every person always has a secret which they may feel scred 또는 embarrassed to share..
Yeah, right. Have 당신 ever noticed how quickly most of those people say it? And how high and squeaky their voices are? Not always, but sometimes. Pfft, give me a break.