Top Definition
When children who have recently acquired a younger sibling feel so excluded that they pretend the new member of the family simply doesn't exist.
Mary: "Who is that?" (motions to John's younger sibling)
John: "What are you talking about? I don't see anything."
Mary: "Oh John, I forgot you are still suffering from new sibling syndrome."
by yepyepyepyepyepyepyep February 02, 2014
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