What is Side-door Friend?
1.
A close friend who feels comfortable coming into your house via the side door, probably off the kitchen. You, too, are comfortable with this friend entering your house through the side door. A side-door friend is probably one of your best friends. Conversely, front-door friends are those acquaintances you are not all that close/comfortable with.
Marcia and me hang out constantly. She is totes a side-door friend.
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