C. F. Keil and Franz Delitzsch on Deuteronomy 23:2

Deu 23:2

So also with the ממזר, i.e., not persons begotten out of wedlock, illegitimate children generally (lxx, Vulg.), but, according to the Talmud and the Rabbins, those who were begotten in incest or adultery (cf. Ges. thes. p. 781). The etymology of the word is obscure. The only other place in which it occurs is Zec 9:6; and it is neither contracted from מוּם and זר (according to the Talmud, and Hitzig on Zec 9:6), nor from זר מעם (Geiger Urschr. p. 52), but in all probability is to be derived from a root מזר, synonymous with the Arabic word “to be corrupt, or foul.” The additional clause, “not even in the tenth generation,” precludes all possibility of their ever being received. Ten is the number of complete exclusion. In Deu 23:3, therefore, “for ever” is added. The reason is the same as in the case of mutilated persons, namely, their springing from a connection opposed to the divine order of the creation.