Can't touch this!
Upon meeting an observant Jew, you may ask, "Are you shomer negiah?" before extending your hand. While the words “shomer negiah” literally mean “observant of touch”, the term refers to someone who refrains from physical contact with members of the opposite sex. Originally known in texts simply as "negiah," the practice generally excludes one's immediate family members--a spouse, children, parents, siblings, and grandparents. There is some debate, however, over the issue of touching siblings after the age of puberty.
The prohibition regarding touch is derived from two verses in Leviticus: "None of you shall come near anyone of his own flesh to uncover nakedness: I am the Lord" (18:6), and "Do not come near a woman during her period of uncleanness to uncover her nakedness" (18:19). Although these verses seem to be directed towards men, women are equally bound by the laws associated with these verses, just as they are with all other negative commandments.
The second of these verses, which prohibits intercourse with a niddah (a menstruating woman), applies not just to one's wife but to any other women as well, married or not (Responsa Rivash 425, Lev. 18:19). The rabbis extend this prohibition to include not just sex, but all touching. And since unmarried women do not go to the mikveh, they are considered to be always in a state of niddah--and therefore always off-limits for sex, or physical contact with men.
Maimonides and Nahmanides, in a well-known rabbinic debate, consider how serious an infraction it is to touch a woman who is a niddah. According to Maimonides in Sefer Hamitzvot, "whoever touches a woman in niddah with affection or desire, even if the act falls short of intercourse, violates a negative Torah commandment" (Lev. 18:6,30). Yet Nahmanides' (1194-1270) commentary states that acts such as hugging and kissing do not violate a negative commandment of the Torah, but only a rabbinic prohibition.
The Siftei Kohen (17th century) further explains Maimonides by stating that he was only referring to hugging and kissing associated with intercourse. There are several places in the Talmud that the Amoraim (talmudic rabbis) hug and kiss their daughters (Kiddushin 81b) and sisters (Shabbat 13a), and their behavior is considered permissible.