I don't understand this whatsoever:
especially the bolded part. What real words does that come from?
Context is that the main character got a cold from staying outside all night; his friend then tells him this.
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is the common colloquial form of:
which is a set phrase that means:
"I told you so.", "Didn't I tell you?", "That's why I told you.", etc.
「やれやれ」 just means "Oh dear!" or something along those lines.