A protest against the grasping covetousness Isaiah 5:8 which is regardless of the rights of the poor upon whose inheritance men encroach (compare the margin reference). The not uncommon reference of the words to the "landmarks" of thought or custom, however, natural and legitimate, is foreign to the mind of the writer.

Remove not the ancient landmark - Do not take the advantage, in ploughing or breaking up a field contiguous to that of thy neighbor, to set the dividing stones farther into his field that thou mayest enlarge thy own. Take not what is not thy own in any case. Let all ancient divisions, and the usages connected with them, be held sacred. Bring in no new dogmas, nor rites, nor ceremonies, into religion, or the worship of God, that are not clearly laid down in the sacred writings. "Stand in the way; and see, and ask for the old paths, which is the good way, and walk therein; and ye shall find rest for your souls;" Jeremiah 6:16. But if any Church have lost sight of the genuine doctrines of the Gospel, calling them back to these is not removing the ancient landmarks, as some have falsely asserted. God gave a law against removing the ancient landmarks, by which the inheritances of tribes and families were distinguished. See Deuteronomy 19:14, from which these words of Solomon appear to be taken.
Even among the heathens the landmark was sacred; so sacred that they made a deity of it. Terminus signifies the stone or post that served as a landmark. And Terminus was reputed a god, and had offerings made to him. Hence Ovid: -
Tu quoque sacrorum, Termine, finis eras.
Fast. lib. i., ver. 50.
Nox ubi transierit, solito celebratur honore,Separat indicio qui Deus arva suo.
Termine, sive lapis, sive es defossus in agroStipes, ab antiquis sic quoque Numen habes.
Te duo diversa domini pro parte coronant;Binaque serta tibi, binaque liba ferunt -
Conveniunt, celebrantque dapes vicinia simplex;Et cantant laudes, Termine sancte, tuas.
Tu populos, urbesque, et regna ingentia finis:Omnis erit, sine te, litigiosus ager.
Fast. lib. ii., ver. 639.
Here we find the owners of both fields bringing each his garland and libation to the honor of this god. They sung its praises, put on its top a chaplet of flowers, poured out the libation before it; and the inhabitants of the country held a festival in its honor. It was, in short, celebrated as the preserver of the bounds and territorial rights of tribes, cities, and whole kingdoms; and without its testimony and evidence, every field would have been a subject of litigation.

Remove not the ancient landmark which thy fathers have set. Or, "the ancient border" or "boundary" (n); by which lands, estates, and inheritances, were marked, bounded, and distinguished; set by ancestors in agreement with their neighbours; which to remove was contrary to a law, and a curse is denounced upon those that did it, Deuteronomy 19:14; and was always reckoned a very heinous crime in early times; See Gill on Job 24:2. This was so sacred a thing among the Romans, that they had a deity which presided over those bounds, and had its name from them. Some apply this, in a political sense, to laws of long standing, and customs of long prescription; and others interpret it, in a theological sense, of doctrines and practices settled by the fathers of the church; which, if understood of Christ and his apostles only, will be allowed; but if of the ancient fathers of the church that followed them, it should not be received; since they were but fallible men, and guilty of many errors and mistakes, both in doctrine and practice.
(n) "terminum antiquum", Pagninus, Junius & Tremellius, Piscator, Michaelis, Schultens.

We are taught not to trespass on another man's right. And it is hard to find a truly industrious man. Such a man will rise. Seest thou a man diligent in the business of religion? He is likely to excel. Let us then be diligent in God's work.

(Compare Proverbs 23:10). Do not entrench on others (Deuteronomy 19:14; Deuteronomy 27:17).

A fourth proverb - a distich - beginning with the warning אל:
28 Remove not the perpetual landmark
Which thy ancestors have set up.
28a = Proverbs 23:10. Regarding the inviolability of boundaries established by the law, vid., at Proverbs 15:25. גּבוּל עולם denotes "the boundary mark set up from ancient times, the removal of which were a double transgression, because it is rendered sacred by its antiquity" (Orelli, p. 76). נסג = סוּג signifies to remove back, Hiph. to shove back, to move away. אשׁר has the meaning of (ὅριον) ὅ, τι, quippe quod. Instead of עולם, the Mishna reads, Pea v. 6, עולים, which in the Jerusalem Gemara one Rabbi understands of those brought up out of Egypt, another of the poor; for "to rise" (in the world) is a euphemism (לשׁון כבוד) for "to come down" (be reduced in circumstances).
(Note: As an analogical example, סגּי נהור, seeing clearly = blind.)