‘Mutual is a well-known trap. The essence of its meaning is that it involves the relation, of x is or does to y as y to x; and not the relation z is or does to z as y to z; from which it follows that out mutual friend Jones (meaning Jones who is your friend as well as mine) and all similar phrases are misuses of mutual. An example of he mistake, which is very common, is: On the other hand, if we [ie the Western powers] merely sat with our arms folded there would be a peaceful penetration of Russia by the country [ie Germany] which was the mutual enemy [ie of both Russia and the Western Powers]. In such places common is the right word, and the use of mutual betrays ignorance of its meaning.My Common Friend would, of course, have a completely different set of associations. Some have been less circumspect.
G. K. Chesterton, in love with the Dickens of the earlier farcical period before he was "gradually absorbed by modern cul- ture and good manners," dwells with "gloomy pleasure" on the "illiterate title," Our Mutual Friend. With "unholy joy" he comments on it, in the introduction to the Everyman edition of this novel, as an indication that Dickens was " still the old self- made man with its disadvantages and its merits." He is no doubt right in his main contention that Dickens was still in 1864 dis- tinctly a man of the middle class, liable to slip in his speech. [Eva M. Campbell 'On the Title, "Our Mutual Friend"' Modern Language Notes 38:4 (1923), 250]My theory is that the title (despite Dickens mischievously suggesting otherwise in the book itself) has nothing to do with this idiom. It's about 'mutuality' in the general, and more forcefully in the commercial and mercantile sense. But I don't have time to go into all that here.