Tumtum (טומטום in Hebrew, meaning "hidden") is a term that appears in Jewish Rabbinic literature and usually refers to a person whose sex is unknown because their genitalia are covered or "hidden" or otherwise unrecognizable genitalia. Although they are often grouped together, the Tumtum has some halachic ramifications distinct from those of the Androgynos (אנדרוגינוס), who has both male and female genitalia.
It is not clear what the actual anatomy of a Tumtum is; however, it would seem that according to medieval commentator Rashi, a Tumtum may have exposed testicles and an unexposed penis.
The Mishnah (Zavim, 2, 1) says that Tumtum and Androgynos have both men's and women's Chumras, meaning that where the law is stricter towards men than women, they are treated as men, but where the law is stricter towards women, they are treated as women.
Tumtum is not defined as a separate gender, but rather a state of doubt. A Tumtum must be either male or female, but since we do not know which one, the strictest gender-dependent obligations or prohibitions are taken on. To this end, positive commandments from which women are exempted are considered binding on a Tumtum.
Nathan ben Jehiel says on his book Aruk (on ערך טם) that the word Tumtum came from the word Atum which means blocked or covered.