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1.
It is a term used to falsely justify apathy on the given subject in the form of a catchy cliche.
A prevalent saying of the people in North America when confronted by the truth.

Me: Did you know that fluoride causes bone brittleness and dental fluorosis?

You: Meh, ignorance is bliss. *swallows the toothpaste*
by youcandothis March 02, 2013
 
2.
The lack of knowlege to a situation. Usually once the whole truth is revealed you realize you were happier being clueless.

Sherry was very happy with her boyfriend.. until she was informed he was cheating on her.
She thought to herself "ignorance is bliss", and broke up with him.
by Snackjuice10 September 16, 2007
 
3.
The lack of knowledge in reference to a situation, particularly a messy one. Once the whole truth is revealed one realizes they were much happier being clueless.

Ana was desperately in love with her long-distance boyfriend Aaron... until she found out he cheated on her multiple times.

Sadly her heart was broken, with no hopes of repair and she realized "ignorance is bliss."

by Elliot.Mars88 December 23, 2007
 
4.
A phrase often uttered by the female persuasion of the mind. Similar to clichés such as “Everything happens for a reason” and “Red rover, red rover. Send Jimmy on over.”. It is often elaborated on in order to describe a distasteful situation in an enlightening manner. As though such a thing were possible.
Examples

Judy - “I was defiled as a young woman and my father believed the boy’s account of the situation over mine. My grandpa went on to say I was just behaving like a stupid whore to other family members and the community at large. I still associate with my family members and will side with them above any outside influence because Ignorance is Bliss.”

Bill - “Judy would never admit to this because she is in an extreme state of self denial and/or Ignorance is Bliss.”

Stephen - “Bill slept with Judy while we were still together and no one has given an honest account of events. Turns out in my case, Ignorance is bliss. Save for the child.”

Child - “Please shoot yourselves in the head and spare my daddy and the following generations to come, who do not operate under moronic idioms such as Ignorance is Bliss.”

Judge - “If Ignorance is Bliss then there would be legal precedent to abolish the family court system. Can I get a ‘Ya!’ up in this biznitch?!”
by someonesafan80 April 05, 2011