stool pegion 線人

stool一詞是指沒有靠背扶手的椅子，如板凳、高腳椅凳等。

a seat without any support for the back or arms:
a bar/kitchen/piano stoola three-legged stool
See also footstool.


也可以指大便：
stool (EXCRETION) noun [C] SPECIALIZED
a piece of excrement:
He told the doctor he had been passing bloody stools.

而比較有趣的是警方的線民：
stool pigeon
noun [C] SLANG DISAPPROVING
a person, often a criminal, who gives information in secret to the police so that they can catch other criminals.

而 pigeon 是指鴿子，俚語則是指容易上當的人，那為何 stool pigeon 會變成線民的意思呢？
應該是背後有個故事吧？

找到答案了：http://www.worldwidewords.org/qa/qa-sto2.htm

Q] From Phil Potter: “I am interested in the historical origins of the phrase stool pigeon.”

[A] These days a stool pigeon is an informer, but when the phrase first appeared—in the US in the 1830s or thereabouts—it meant a person used as a decoy to entice criminals into a trap. In that sense, it’s not far from the French agent provocateur.

Most modern dictionaries say the phrase came from the practice in hunting of tying or nailing a dead pigeon to a stool to act as a decoy. Why a pigeon? More importantly, why a stool? Presumably the reference is to a tree stump rather than the backless seat with short legs (though as the tree stump sense of the word is rather uncommon, it does make one wonder; would a hunter actually carry a stool out into the field specifically for the purpose?). To add to my suspicions about this definition, it seems from the evidence that stool pigeon has always been used for a person, never for a hunter’s decoy.

The phrase starts to make sense when you delve into the history of words for decoys, of which there are a surprising number. The one to focus on is the archaic term stale, which probably comes from the French estale, applied to a pigeon (aha!) used to entice a hawk into a net. Stale appears in English from the early fifteenth century; by the end of the following century it was being used for a person who acted to entrap another.

Another spelling was stall. At the end of the fifteenth century this began to be recorded as a bit of thieves’ jargon for a pickpocket’s accomplice, who acted as a decoy to distract the attention of the victim. The verb for this action evolved into our modern sense in phrases like to stall for time.

It seems pretty clear from all this that the Americans who started to employ stool for a decoy bird were using yet another version of this old word. The use of stool in this sense is older than that of stool pigeon—the earliest reference in the OED is to the town records of Huntington, New York, in 1825: “No person shall be permitted to gun with macheanes [machines] or stools in said Town”. It was also a verb: stooling was decoying ducks or other birds by the use of stools.

The other half of the expression, pigeon, has been used in slang since at least the sixteenth century for a person who allows himself to be swindled, a simpleton or fool, a sucker. It seems that this idea formed part of the genesis of stool pigeon, so you might explain the term as “a fool used as a decoy”, though with a nod to the literal sense of the word. By the 1840s, stool pigeon had shifted from being a decoy to being an informer.