kakōs

This is the ordinary Greek adverb for "badly", and the Gospel writers use it most often about bodies. Joined to the verb "to have", it makes the common phrase for being ill, which reads literally as "having badly". The phrase names no disease and offers no diagnosis. It says only that a person is doing badly, and the surrounding words supply the details: a fever, a lifelong disability, or a servant close to death.

Because the word measures how bad something is rather than what it is, it also works as a plain intensifier. In two of Matthew's healing stories it stretches an affliction to its worst: a daughter is not merely troubled by a spirit but badly troubled, and a boy does not merely suffer but suffers badly.

One use turns the same word back on people rather than on their bodies. In the parable of the vineyard the tenants pronounce their own sentence: bad men, and they will come to a bad end. Greek liked that pairing of the adjective with its adverb, and the effect is a grim rhyme rather than a legal verdict.

Almost every occurrence sits in the healing summaries of Matthew, Mark and Luke, in the lines where crowds carry "all who were doing badly" to Jesus. That is why the word feels medical in English, though the Greek itself is entirely plain.

The same adverb turns moral when it describes an action rather than a body. Something done badly here is something done in a way it should not have been done, and the fault can lie in the deed, in the manner, or in the motive behind it.

When Jesus is struck at his hearing he sets two adverbs side by side: if he has spoken badly, let the man produce the evidence, but if he has spoken well (καλῶς), why the blow? The contrast is the whole point, and it is a contrast of right and wrong, not of skill. James uses the word of prayer in the same way. People ask and do not receive, he says, because they ask badly, and what he then names is not a flaw in their words but the pleasure they mean to spend the answer on.

One use is narrower still. In Greek, putting this adverb with a verb of speaking and naming a person means insulting or cursing that person, and Paul quotes exactly that expression when he takes back his outburst against the high priest. English keeps a trace of the same turn of phrase in "to speak ill of" someone.