This is a proverbial expression, found also in classic and Rabbinical authors. We know of no custom alluded to in this proverb save the general habit of giving with the right hand, as it is more conveniently used than the other; but Mr. Jowett speaks of a custom lie noticed in Palestine, which, if it existed in our Lord’s time, might have suggested the saying of the text. In giving an account of his visit to Nablous, Mr. Jowett says: “The manner in which the Samaritan priest desired me, on parting, to express our mutual good-will, was by an action, than which there is not one more common in all the Levant. He put the forefinger of his right hand parallel to that of his left, and then rapidly rubbed them together, while I was expected to do the same, repeating the words, ‘right, right;’ or, in common acceptation, ‘together, together.’ It is in this manner that persons express their consent on all occasions: on concluding a bargain, on engaging to bear one another company, and on every kind of friendly agreement or good understanding” (Christian Researches, p. 209).
The idea of the text may be, that alms-giving is not to be a matter where the hands are put together in token of an understanding with someone else, but it is to be done privately.