"I'm as thick as a plank"

This quote by Princess Diana is a funny method of expressing her absence of intelligence. She is implying that she is not really smart and is not able to comprehend or understand things quickly. This might be translated as a self-deprecating joke, as she is making fun of her own intelligence. It could also be interpreted as a way of revealing her humbleness, as she is not attempting to boast about her intelligence or knowledge. It could likewise be translated as a way of expressing her vulnerability, as she is admitting that she is not the most experienced individual and is open to gaining from others. In general, this quote is an amusing way of expressing her lack of intelligence and humbleness.