Besides Lev 20:20-21, the only other uses of the Hebrew word are in Gen 15:2, where Abraham complains that he is childless, and Jer 22:30, where it must mean that none of Jeconiah's children would succeed him as king, because Jeconiah did have a son (1Chron 3:17).
There is also the apocryphal Sirach 16:3, which is written in Hebrew ~200 BC
1 Do not desire a multitude of useless children, nor rejoice in ungodly sons.
2 If they multiply , do not rejoice in them, unless the fear of the Lord is in them.
3 Do not trust in their survival, and do not rely on their multitude; for one is better than a thousand, and to die childless is better than to have ungodly children.
I would say that in Lev 20 it means literally childless as in Gen 15 & Sirach, and that the prophecy in Jer 22 is figurative.
The answer I believe is more likely to be of the surrounding context of Leviticus 20, that is if sin which produce offspring or children, adultery and incest, as well as the abuse of the generative action in bestiality and homosex.
All of these laws are sexual in nature, and in the cases in Leviticus 20:20&21 the specific adultery is that of brother with sister in law and nephew with aunt by marriage.
Be cause the Law calls for these sinners to be put to death the question naturally arises , "what of the children?"; In the biblical society this is of different impact than perhaps in our modern thinking, namly, be cause of the law of levirate marriage from
5 If brethren dwell together, and one of them die, and have no child, the wife of the dead shall not marry without unto a stranger: her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of an husband's brother unto her.
This law like inheritance in Israel extends to family beyond brother and to uncle and other 'near kin' :
Rth 3:2 And now is not Boaz of our kindred, with whose maidens thou wast? , Rth 4:1;
Rth 4:8 Therefore the kinsman said unto Boaz, Buy it for thee. So he drew off his shoe.
Thus in the Israelite society a brother or uncle's wife bearing a child allows for the question of, "should the child of this union be born before capital punishment and be adopted as family or shall the child die in-utero with the sinful party of brother and uncle?
And for that reason, not unlike abortion adjudication of the present day, in these cases YHWH God needed to be specific as to how punishment should proceed under these conditions.
Leviticus 20: 10 And the man that committeth adultery with another man's wife, even he that committeth adultery with his neighbour's wife, the adulterer and the adulteress shall surely be put to death.
11 And the man that lieth with his father's wife hath uncovered his father's nakedness: both of them shall surely be put to death; their blood shall be upon them.
12 And if a man lie with his daughter in law, both of them shall surely be put to death: they have wrought confusion; their blood shall be upon them.
13 If a man also lie with mankind, as he lieth with a woman, both of them have committed an abomination: they shall surely be put to death; their blood shall be upon them.
14 And if a man take a wife and her mother, it is wickedness: they shall be burnt with fire, both he and they; that there be no wickedness among you.
15 And if a man lie with a beast, he shall surely be put to death: and ye shall slay the beast.
16 And if a woman approach unto any beast, and lie down thereto, thou shalt kill the woman, and the beast: they shall surely be put to death; their blood shall be upon them.
17 And if a man shall take his sister, his father's daughter, or his mother's daughter, and see her nakedness, and she see his nakedness; it is a wicked thing; and they shall be cut off in the sight of their people: he hath uncovered his sister's nakedness; he shall bear his iniquity.
18 And if a man shall lie with a woman having her sickness, and shall uncover her nakedness; he hath discovered her fountain, and she hath uncovered the fountain of her blood: and both of them shall be cut off from among their people. note
19 And thou shalt not uncover the nakedness of thy mother's sister, nor of thy father's sister: for he uncovereth his near kin: they shall bear their iniquity. note
20 And if a man shall lie with his uncle's wife, he hath uncovered his uncle's nakedness: they shall bear their sin; they shall die childless.
21 And if a man shall take his brother's wife, it is an unclean thing: he hath uncovered his brother's nakedness; they shall be childless. note
22 Ye shall therefore keep all my statutes, and all my judgments, and do them: that the land, whither I bring you to dwell therein, spue you not out.