banishment (n.)

late 14c., banischen, "to condemn (someone) by proclamation or edict to leave the country, to outlaw by political or judicial authority," from banniss-, extended stem of Old French banir "announce, proclaim; levy; forbid; banish, proclaim an outlaw" (12c., Modern French bannir), from a Germanic source (perhaps Frankish *bannjan "to order or prohibit under penalty"), from Proto-Germanic *bannan (see ban (v.)). The French word might be by way of Medieval Latin bannire, also from Germanic (compare bandit). The general sense of "send or drive away, expel" is from c. 1400. Related: Banished; banishing.

To banish is, literally, to put out of a community or country by ban or civil interdict, and indicates a complete removal out of sight, perhaps to a distance. To exile is simply to cause to leave one's place or country, and is often used reflexively: it emphasizes the idea of leaving home, while banish emphasizes rather that of being forced by some authority to leave it .... [Century Dictionary]

common suffix of Latin origin forming nouns, originally from French and representing Latin -mentum, which was added to verb stems to make nouns indicating the result or product of the action of the verb or the means or instrument of the action. In Vulgar Latin and Old French it came to be used as a formative in nouns of action. French inserts an -e- between the verbal root and the suffix (as in commenc-e-ment from commenc-er; with verbs in ir, -i- is inserted instead (as in sent-i-ment from sentir).

Used with English verb stems from 16c. (for example amazement, betterment, merriment, the last of which also illustrates the habit of turning -y to -i- before this suffix).

The stems to which -ment is normally appended are those of verbs; freaks like oddment & funniment should not be made a precedent of; they are themselves due to misconception of merriment, which is not from the adjective, but from an obsolete verb merry to rejoice. [Fowler]