백정 originally referred to "commoners" or "people (mainly, farmers) without 정인's duty (e.g. military service)" according to this:

백정: 고려시대 군역(軍役)·기인역(其人役)·역역(驛役) 등의 특정한 직역(職役)을 부담하지 않고 주로 농업에 종사하던 농민층.

백정이라는 명칭은 원래 중국의 남북조와 수(隋)나라에서는 무관자인 평민, 즉 백성을 일컫던 말이다.

백정의 ‘백(白)’은 ‘없다’ 또는 ‘아니다’라는 의미를 지닌 말이고, ‘정(丁)’은 ‘정호(丁戶)’ 또는 ‘정인(丁人)’이라는 뜻이므로 백정은 정호(정인)가 아닌 사람을 지칭한다.

군역 means the duty of military service. 백정 had other duties, too, but it does not mean that they had no duty of paying taxes.

As SanghyunPark mentioned, 백정 had referred to one who is not 정호 or 정인. Michaelyus correctly guessed the meaning of 백(白). You assumed that 백정 meant "an unfit person"; however, that is incorrect.

According to a publication (written in English), the citizens called 백정 (Bai Ding) were mostly composed of land-holding farmers. Thus, 백정 did not mean "a person without land." Please read it from "the common citizens" to "legal privileges" on page 69.

백정 originally referred to "commoners" or "people (mainly, farmers) without 정인's duty (e.g. military service)" according to this:

백정: 고려시대 군역(軍役)·기인역(其人役)·역역(驛役) 등의 특정한 직역(職役)을 부담하지 않고 주로 농업에 종사하던 농민층.

백정이라는 명칭은 원래 중국의 남북조와 수(隋)나라에서는 무관자인 평민, 즉 백성을 일컫던 말이다.

백정의 ‘백(白)’은 ‘없다’ 또는 ‘아니다’라는 의미를 지닌 말이고, ‘정(丁)’은 ‘정호(丁戶)’ 또는 ‘정인(丁人)’이라는 뜻이므로 백정은 정호(정인)가 아닌 사람을 지칭한다.

군역 means (the duty of) military service. 농민층 means the class of farmers. 백정 also had duties such as paying taxes.

As SanghyunPark mentioned, 백정 had referred to one who is not 정호 or 정인. Michaelyus correctly guessed the meaning of 백(白). You assumed that 백정 meant "an unfit person"; however, that is incorrect.

According to a publication (written in English), the citizens called 백정 (Bai Ding) were mostly composed of land-holding farmers. Thus, 백정 did not mean "a person without land." Please read it from "the common citizens" to "legal privileges" on page 69.