Referring to the testicles, and the sperm therein, of a man who has one or more children.
While sitting on the floor changing my daughter's diaper, she's started flailing her legs around and kicked me in the 'brothers and sisters'.
by vankinney September 2, 2009
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When you like a member of the opposite sex. Eventually wanting to be in a relationship with them, but they shut you down with the "I love you like a brother/sister"
Brother/Sister Zone:

Guy: Derpina, will you go out with me?
Girl: Derp, I'm sorry, but I love you like a brother that I've never had, and I don't want to ruin that!
by tinkerbelleh_01245 June 12, 2012
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A half sibling via your father

ie from his 'spunk donation'

usually applying to promiscuous men with more than one family.
I didn't know you had a brother/sister - Oh he's my "Spunk Brother / Sister"

Underlining the fact that the relationship is not a close one.
by Big Superfuzz April 2, 2009
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A very close friend you trust a lot
When a brother/sister (friend) likes another so much and trusts them
by MasterOfStats January 2, 2019
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My enemy.
- My brother/sister is as lank as a twig.
by Spacet September 3, 2022
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If everyone was a thought brother/sister, there wouldn't be anyone more in charge than the next, there would be no hierarchy.
A thought brother/sister must be a dream to wake up from, since people seem to want a thought leader more than they want one of those.
by The Original Agahnim February 2, 2022
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The actor/singer/celebrity that could be the little brother or sister of anyone with a little brother or sister, since this person actually was somebody's little brother or sister growing up.
If the singer/actor thought his/her boy/girl next door image was bad, he/she could have always got stuck with the America's little brother/sister image of the other actor/singer/pop star.
by The Original Agahnim September 29, 2021
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