deregister v.

active word-forming element in English and in many verbs inherited from French and Latin, from Latin de "down, down from, from, off; concerning" (see de), also used as a prefix in Latin, usually meaning "down, off, away, from among, down from," but also "down to the bottom, totally" hence "completely" (intensive or completive), which is its sense in many English words.

As a Latin prefix it also had the function of undoing or reversing a verb's action, and hence it came to be used as a pure privative — "not, do the opposite of, undo" — which is its primary function as a living prefix in English, as in defrost (1895), defuse (1943), de-escalate (1964), etc. In some cases, a reduced form of dis-.

late 14c., registre, "public record book, private account book, an official written account regularly kept," from Old French registre (13c.) and directly from Medieval Latin registrum, regestrum, properly regestum, from Late Latin regesta "list, matters recorded," noun use of Latin regesta, neuter plural of regestus, past participle of regerere "to record; retort," literally "to carry back, bring back" from re- "back" (see re-) + gerere "carry, bear" (see gest).

With unetymological second -r- in Medieval Latin and Old French by influence of other Latin nouns in -istrum (French -istre). The word was also borrowed in Dutch, German, Swedish, Danish.

Some later senses seem to be influenced by association with unrelated Latin regere "to rule, to guide, to keep straight." Meaning in printing, "exact alignment of presswork" is from 1680s. Musical sense is from 1811, "compass or range of a voice or instrument," hence "series of tones of the same quality" (produced by a voice or instrument).

From mid-15c. as "a record-keeper, recorder;" sense of "device by which data is automatically recorded" is by 1830, from the verb.