Definition
Mittens connected by yarn or string (idiot string (Canadian English)) running through one sleeve, along the back and out at the other sleeve of a coat, to prevent the mittens becoming lost. Generally worn by small children.
How People Actually Use It
(noun) Someone who is extremely stupid and forgetful. Refers to ancient practice of attatching two mittens to a long string and then placing each through opposite sleeves on your coat so that when you remove the mittens you do not lose them.
"Wife: "Happy anniversary honey!" Husband: "Oh, that's right, today is our anniversary" Wife: "Idiot mitten!""
Source: Urban Dictionary