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1.
A person between aged 20-30 who is living in their parents house when they are old enough to live in his own apartment. I derived the term from the word "house-wife." A house-wife lives at home and spends her husband's money on silly things like clothes and jewelery. Likewise, a house-child (or house-daughter in this case) lives for free and spends her father's money on the same things.

A house-child, by definition, must be:
1) old enough to be a house-wife
2) unemployed
3) have lived away from the house for a period of time (i.e. college, grad school) and is now coming back to the house just when her parents thought they were finally rid of her!!
4) so busy house-childing that she frequently requests an assistant.
5) too old to live at home. Someone who is of normal age to live at home (i.e. a high-school student) cannot be a house-child.

Note: house-childing is becoming an increasingly popular activity given that most people in their 20s can no longer afford rent due to the current economic downturn
house-childing, verb
house-child, noun

Person A: "So Rebecca, now that you have graduated with your Masters degree....what are you up to these days?"
Person B: "I'm house-childing. It's a full-time job."

Person A: "I'm a real estate broker and my wife is a dental hygienist. What you do, Rebecca?"
Person B: "I'm a house-child. It keeps me very busy."
by R-to-the-EBECCA November 30, 2009
 
2.
An adult who does not work and resides with their parents. Being a housechild is similar to being a housewife (or househusband) except you don't have to perform gross sexual acts that you don't want to on the people or person who supplies you with money, housing, etc.
Lucy: Dave asked me to do the nastiest thing last night.
Amy: Did you do it?
Lucy: I didn't have a choice! He refused to give me any more money until I- I... I don't even want to talk about it.
Amy: Wow, I'm glad I'm a housechild instead of a housewife!
by Pinkie1 December 25, 2013