It has been said that the custom of shaking hands originated as a means of demonstrating that one is not concealing any weapons. While most people no longer worry about their contemporaries carrying weapons, for traditional Jews, this disarming gesture can be a unique social challenge.

The halachic concept is known as being shomer negiah, a guardian of touch. Those who are shomer negiah are careful not to touch people of the opposite gender, with the exception of immediate family (parents, grandparents, spouses, children, grandchildren and siblings).

The laws of shomer negiah are distinctly elucidated in the Rambam’s (Rabbi Moses ben Maimon, 12th century Spain) Mishnah Torah:

Whoever shares physical intimacy with one of the arayot [those with whom sexual relations would be illicit] without actually becoming involved in sexual relations or embraces and kisses [one of them] out of desire and derives pleasure from the physical contact should be lashed according to Scriptural Law. [This is derived from Leviticus 18:30 which] states: ‘To refrain from performing any of these abominable practices,’ and [ibid.:6 which] states: ‘Do not draw close to reveal nakedness.’ Implied is that we are forbidden to draw close to acts that lead to revealing nakedness” (Issurei Biah 21:1). 

Refraining from any physical contact, even such an innocuous act as tapping another person on the shoulder, is protection from drawing close to acts that lead to revealing nakedness.

Society today is quite desensitized to the power of touch. And while there is some room for leniency on insignificant touching (“Even if he is young, lust is not stirred by a momentary act” – Jerusalem Talmud Sotah 3:1), one can never know the thoughts of the other person. What may seem like an innocent handshake between a man and a woman, can result in sending an unintended message to either person.