Every Idle Word

It is not just every word that intentionally conveys malice or selfishness that Jesus says will demand an account in Matthew 12:36, but every idle word.  Misspeaking when one meant to say something else is not irrational or otherwise sinful (Deuteronomy 4:2), so that could not be the issue at hand.  Idle does not mean accidental; it means something more like careless here, as in every carelessly spoken word that violates actual moral obligations.  What are some of the Bible's condemnations when it comes to speech?  First of all, profanity is wholly nonsinful by Biblical standards unless it is directed at a person with the intention to degrade them (James 3:9 would apply here), as the aforementioned verse from Deuteronomy would necessitate given that the Bible nowhere condemns profanity, so casually uttering expletives without paying much attention to one's words in the moment would not at all be what Jesus is warning against in Matthew 12.

As for actual verbal sins, which are of course in general far lesser than sins that involve the greater kinds of physical harm of the beings made in the image of God, things like cursing God (Leviticus 24:16) and cursing one's parents (Exodus 20:17) are two examples of verbal sins that are prescribed the death penalty, meaning they are particularly depraved by comparison to many other verbal sins like minor slander or lies.  Of course, all slander is condemned fairly early in Mosaic Law in the Ten Commandments (Exodus 20:16), and then there is the universal prohibition of lies (Leviticus 19:11).  Moreover, the same reason why degrading physical treatment--either treatment with the intent to dehumanize or that goes beyond a fixed point specified in Deuteronomy--is Biblically evil (Deuteronomy 25:1-3) would apply to words as well.  Lastly, since one's gender, race, descent, age, and physical wholeness or beauty have nothing to do with a person's status as a bearer of God's image (Genesis 1:26-27), any sexist, racist, or ageist comments would be sinful by default as well, though this overlaps with lies and slander since such comments are used as if it was true that these things determine a person's worth, character, or philosophical competence.

With these verbal sins of the Bible actually recognized, then it becomes clear which words Jesus would be condemning even if one only speaks them idly, the term otherwise being carelessly or apathetically.  Exhaustion, emotional distress, extreme desire, and harsh situations might make the difference between purely malicious, selfish, or irrational intentions and careless words spoken out of lesser self-awareness, but they do not exempt someone from whatever moral obligations they have with their words, motivations, beliefs, and deeds.  To commit verbal sins is still immoral even if a person almost thoughtlessly (yes, all decisions, perceptions, intentions, and other mental activities are thoughts in the broadest sense of the word, but I mean by thoughtless that there is a lack of rationalistic care) indulges in them.

Every malicious, slanderous, degrading, or deceptive word would be the kind condemned by the Bible, and thus idly speaking such words would be the sin in view by Jesus in Matthew 12.  What makes a word malicious or degrading or so on is not how the recipient feels, though--not only is a subjective feeling not an indicator of the objective nature of something else and the concept of harshness not always the same as the concept of injustice, but Jesus himself, the very person promising that every idle word will be enough to condemn humans, speaks harshly to some in the gospel accounts, with very direct, controversial tirades against hypocrisy, distortion of Mosaic Law, and the arrogance and stupidity of looking to tradition.  The Jesus of the actual Bible is neither verbally abusive nor in any way someone who practices tolerance of selfish people in word or deed.

What his own point in Matthew 12:36 would be is not that all harsh or subjectively unpleasant words are evil, but that there is no excuse for even the slightest of verbal sins, trivial though they might be in comparison to others.  The entire category of verbal sins is almost always of a lesser depravity than physical mistreatment, yes.  It is just that being less immoral does not make something justifiable or good.  Throughout a person's life, he or she might experience moments or even much longer periods where it is easier to say words, even ones that might be morally charged, without caring about or reflecting on (prior or in the moment) the intended meaning, or how applicable that intention is to a situation or other person.  There is never a time when not sinning is beyond anyone's ability, contrary to evangelical ideology, even with something as simple as words.  The comment of Jesus in the verse after his caution with idle words even predicts quite the stakes for this (Matthew 12:37): "For by your words will you be acquitted, and by your words will you be condemned."