A bastard in Torah Judaism is not defined by being born out of wedlock: a bastard is instead the product of a truly forbidden relationship. The classic example of a bastard, a mamzer, are the grandsons of Lot, produced through his incestuous encounters with his daughters. (D. 23:3-4)
The word itself can linguistically be used to explain the meaning of what a mamzer is. The core word of mamzer is zar, which is a rim. And like everything else in the Torah, the meaning of a rim, a zar, is full of symbolism.
As explained here, the tabernacle had rims on various items – and they were used to mark the boundary between things that could have a divine connection, and things that could not.
Outside of the tabernacle, zar is explained through examples:
And they shall eat those things with which atonement was made to consecrate and to sanctify them: but a stranger (zar) shall not eat of them, because they are holy. (E. 29:33)
You shall offer no strange (zar) incense on it, nor burnt sacrifice, nor meal offering; neither shall you pour drink offering upon it. (E. 30:9)
Whoever compounds any like [the annointing oil], or whoever puts any of it upon a stranger (zar), shall even be cut off from his people. (E. 30:33)
And Nadav and Avihu, the sons of Aharon, took each of them his censer, and put fire in it, and put incense on it, and offered strange (zar) fire before the Lord, which He commanded them not. (L. 10:1)
And when the tabernacle sets forward, the Levites shall take it down: and when the tabernacle is to be pitched, the Levites shall set it up: and the stranger (zar) that comes near shall be put to death. (N. 1:51)
Zar is used to refer to things that are not allowed to be in contact with holiness! We have other words for “stranger” or “non-Jew” in the Torah. But this word, zar, is uniquely used as something that must be separate from holy things.
How does this connect to a mamzer? Because the language – and thus the symbolism – are parallels!
No mamzer shall be admitted into the congregation of G-d; no descendant of such, even in the tenth generation, shall be admitted into the congregation of G-d. (D. 23:3)
As we have shown before, a mem prefix tells of the essence of a thing: Mayim (water) is the essence of Yam (water), and Mikdash(tabernacle) is the essence of Kodesh (holiness). So how or why is a mamzer definitionally the essence of zar?
Because the mamzer was created as an antithesis of holy relationships. Lot’s daughters seduced their father to create children with a single-minded desperation, as if they had fallen off the rim of theworld. They, who were at the very edge of Avraham’s family, acted at the edge of habitation (in a cave), at the edge of existence (in a place with no other people), in a time when they were not sure the world had ended, at the very edge of darkness. A mamzer is what is produced when people are far, far beyond the behaviors G-d expects of mankind. Every element of a mamzer stands against the principles of holy and spiritual relationships between man and woman, and man and G-d.
Mamzer is spelled differently from mayim or Mikdash or mikneh or many other words where mem is a prefix, telling us of the essential quality. This word uniquely has two mems.
Why?
I think the answer is found in the injunction:
No mamzer shall be admitted into the congregation of G-d; no descendant of such, even in the tenth generation, shall be admitted into the congregation of G-d. (D. 23:3)
A zar is a boundary in space, denoting the line between the sacred and the profane. But a mamzer is not just bound by space, he or she is also bound by time! A mamzer is forbidden for ten generations (or even in perpetuity, male descendants of Ammon and Moab (D. 23:4)).
So the first mem denotes that a mamzer is at the essence of what cannot be in contact with G-d in space. And the second mem tells us that the mamzer cannot be in contact with G-d in time.