Queen Elizabeth I, though known as the virgin queen, had many suitors and lovers. One of them being Francis, the Duke of Anjou. Of all her lovers, Queen Elizabeth was most fond of Francis and actually planned on marrying him at one point. Though, having second thoughts, she refused his proposal and shortly after ended their relationship. She may have refused their marriage, but when Francis left for France, she was actually quite upset. She wrote the poem “On Monsieur’s Departure” for him.
Queen Elizabeth often used unflattering slang names for her closest friends, such as “pygmy” for Robert Cecil, and “frog” for Francis. Some believe this nickname was given to him because of the frog-shaped earring Francis had given her.
Headcannon: Arthur, who was extremely impressed by his queen and looked up to her, sees the blooming romance between the couple. He notices the nickname she gives him and picks up on it.
He starts calling France this, believing that the term is endearing. But France just ends up taking offense to it to England’s confusion. (The term also being rather derogatory at the time) Though Arthur may still use this phrase when frustrated with Francis, this nickname is nonetheless affectionate.