I use the phrase
How long is a piece of string
every now and then, usually when somebody asks me when I'll finish some programming task that I haven't even looked into yet.
I know what it means:
How long is a piece of string?
1. (colloquial, often humorous) Used as a response to a question such as "How long will it take?" or "How big is it?" when the length or size is unknown, infinite, or variable.
-How long is a piece of string?, Wiktionary.
But I'd like to know what is the origin of this phrase?
Why is it used to denote an unknown size, length or time? As in, what is it about the string that makes it supposedly hard or impossible to measure?