dis- Also dif-.
Expressing a variety of negative senses.
[Latin dis-, sometimes via Old French des-.]
The prefix had various linked senses in Latin, such as reversal, moving apart, removal or separation; sometimes it could express simple negation. For example, in dissuade it indicates reversal, as the stem verb suadere means to advise or persuade; in dissipate, from supare, to throw, it has the sense of ‘apart’, so literally ‘to scatter’; in discharge, from Latin discarricare, to unload, it signals the opposite of carricare, to load.
As a prefix in English, it can have any of these senses. It indicates reversal in disaffirm, disconnect, disappear, disembark, disestablish, disown, dispossess, disqualify; removal in disbud, disburden, disembowel; negation in disability, dishonour, dislike, dissimilar.
The prefix is active, for instance in recent decades forming disambiguate, to remove the ambiguity from some situation; disintermediation, eliminating intermediaries from a chain of suppliers or traders; disinformation, false information which is intended to mislead.
In a few examples in Latin, dis- could intensify the action of the stem. In English, a few examples exist, but they are unusual: disannul, to make null and void; dissever, to divide or sever something.
Before stems beginning with f, the prefix became dif- in Latin, examples of which in English include differ, difficulty, diffident, diffract, and diffuse.
Though dismal looks like a member of this set, it actually derives from medieval Latin dies mali, evil days.