Douglas Harper's Etymology Dictionary
"liable to 'take a miff,' " 1810, from miff (n.) + -y (2). Related: Miffiness.
Wiktionary
a. Easily irritated.
Wikipedia
Miffy (, pronounced ) is a small female rabbit in a series of picture books drawn and written by Dutch artist Dick Bruna. The original Dutch name, Nijntje, is a shortening of the diminutive konijntje, "little rabbit".
The first Miffy book was produced in 1955, and almost 30 others have followed. In total they have sold over 85 million copies, and led to two separate television series as well as items such as clothes and toys featuring the character. A feature-length film, Miffy the Movie, was released on 30 January 2013.
Two television series based on the character have been produced: Miffy and Friends, which ran from 2003 until 2007, and Miffy's Adventures Big and Small, which premiered in 2015.