Can two persons be called mutual friends? Does not the word of necessity imply three or more than three? (See the controversy in Notes and Queries June 9, 1894, p. 451.)
A mutual flame was quickly caught, Was quickly, too, revealed; For neither bosom lodged a thought Which virtue keeps concealed.
Edwin and Emma.
(Mutual = reciprocal.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894