Moistism is derived from the term wet blanket. One may moisten a conversation if they choose to add a moistism to a social conversation.
Person A: "There was like 100 people there last night".
Person B: Actually it was only 96.
Person A: Wales is one of my favourite countries.
Person B: Actually Wales is part of the UK or Great Britain and is not recognised as a country in it's own right.
Person A: I found out I have AIDS and it's going to kill me.
Person B: Actually AIDS doesn't kill you it attacks your immune system making you more susceptible to colds and virus'. It's actually those that kill you.
Person B: F*** you.