The term 'informant, informally' can be summarized by two crossword solutions: 'STOOL' (5 letters) and 'stoolpigeon' (11 letters). A STOOL is a type of chair, but it is also used as a slang term for an informant. A stoolpigeon is another slang term for an informant, which originated in the early 20th century. It is derived from the phrase 'perching on a STOOL, ' which was used to describe someone who was eavesdropping or spying on others.