n. — informal, Clothing
a string that attaches small objects, usually mittens but also tools or other small items, to clothing to prevent loss (see Image 1).
Type: 5. Frequency — Idiot string is most frequent in Canada according to internet domain searches (see Chart 1). The term is more affectionate than unkind, since the string helps small children keep track of their mittens; sometimes mittens attached by a string are called idiot mittens (see the 2011 quotation). The term is a preservation in the sense of using an outdated term, i.e. idiot, from a different era.
See also COD-2, s.v. "idiot string", which is marked "Cdn', while ITP Nelson and Gage-5 do not list the term.
- Sometimes used in the plural, see, e.g. the 2000 quotation.
Images:

Image 1: Mittens attached by an idiot string. Source: Wikimedia Commons. Photo: Bin im Garten
Chart 1: Internet Domain Search, 14 Aug. 2012