"Others" here is referring to another person; it could even refer to a stranger. What Solomon is giving us in this section (verses 15-29) is counsel for balanced living.
Verse 20 shows that no one on earth does what is right all the time, never making a mistake. It is the character of a just man to do good, but that is not what always happens.
Then verse 21 begins with the word "also," which means "in addition," "likewise," "too," "in like manner," and "furthermore," suggesting that verses 21-22 continue the thought of verse 20. In just about every situation, sin is involved. Either we have sinned or others have sinned against us—or both.
Solomon advises us not to pay attention to or take to heart everything people say, even if we hear an employee or someone under our authority insulting us—because we know that we have insulted other people many times.
Understanding the word "curse" is important here. It does not mean "to invoke or bring evil or misfortune upon" or "to damn." It is the Hebrew word qalal, which means "to make light, trifling, bring into contempt, abate." Our English word abate means "to make less," "to reduce in quantity, value, degree, or intensity," "to beat down," and even "to deprive."
These verses do not give specific examples of what might have been said. Perhaps it was a defaming remark, an unwarranted comment, an angry threat, a joke at another's expense, or deliberate untruths. What was said is ultimately unimportant.
Baptist commentator John Gill (1697-1771) writes in his Exposition of the Old Testament on verse 21:
Seeing so it is, that imperfection attends the best of men, no man is wise at all times, foolish words and unguarded expressions will sometimes drop from him, which it is better to take no notice of; they should not be strictly attended to, and closely examined, since they will not bear it. A man should not listen to everything that is said of himself or others; he should not curiously inquire what men say of him; and what he himself hears he should take no notice of; it is often best to let it pass, and not call it over again; to feign the hearing of a thing, or make as if you did not hear it; for oftentimes, by rehearsing a matter, or taking up words spoken, a deal of trouble and mischief follows.
In the face of provocation, the true quality of self-restraint is displayed in our ability to take it patiently with forbearance and longsuffering. A person who is longsuffering is not quick to retaliate or promptly punish someone who has insulted, offended, or harmed him.