If you cannot make up you mind which of two things to do, you are liable to get yourself into difficulties by doing neither. If a car comes suddenly round the corner and you cannot decide which way to jump, the car will hit you, particularly if the driver is suffering from a similar indecision!
Dr Brewer explained that the allusion is to a practical joke (called Ambassador’) played on greenhorns at see. ‘A tub full of water is placed between two stools, and the whole being covered with a green cloth, a sailor sits on each stool, to keep the cloth tight. Two sailors represent Neptune and Amphitrite, and the greenhorn, as ambassador, is introduced to the majesties. He is given the seat of honour between them; but no sooner does he take his seat than the two sailors rise and the greenhorn falls into the tub, amidst the laughter of the whole crew.’