Everything but the kitchen sink is what you brought with you when you included almost everything you could think to bring. Example: "Wow, your suitcase is huge. What do you have in there? Everything but the kitchen sink?"

When we we are deciding what to bring with us, we try to take everything possible. But some things, like the kitchen sink, are just not convenient. Example: "Every time we go camping my wife packs everything but the kitchen sink."

Everything but the kitchen sink means that you included almost everything you could think to include. Example: "My dad's new luxury car is great. It has everything but the kitchen sink."