My great-grandmother on my father's side was Pennsylvania Dutch, straight from Amish country. While she lived in Arizona for most of her adult life many of the colloquial and familiar phrases from her childhood remained in her vocabulary. I grew up with those phrases being commonplace. My grandmother, her daughter used them, and then by extension my mother started using them. As small children my mother often took us over to see my great-grandmother and I have fond memories of time spent there.
One of the phrases she used often was "Oh, don't go getting your butt up over the dashboard" anytime someone would get a little put off or upset. It was a phrase my mom adopted and eventually as an adult I adopted as well (since we weren't allowed to say "butt" as children or teens for that matter). Anyway, early on in our relationship I used that phrase in front of my husband and I was dead serious when I said it. Needless to say, my husband was rolling in laughter. He had never heard the phrase and to this day when I use it, which is rare, he finds it very humorous. I guess because it has been a part of my life since childhood I don't see why it is so funny.
Do you have any particular phrases that are specific to just your family or small circle of friends?