A mental tool to make a connection between one idea and another, specifically for simple ideas that only a donkey would need help remembering. A mnemonic is an example.
Professor: B-cells mature in the Bone marrow while T-cells mature in the Thymus. B with B and T with T. That's a simple donkey bridge for you to remember when studying the immune system.
Student: What the crap is a donkety bridge?
Professor: (see definition)