"divide and assign according to rule," 1570s, from French apportionner, from Old French aporcioner "apportion, share out," from a- "to" (see ad-) + portioner "to divide into portions," from portion "share, portion" (see portion (n.)). Related: Apportioned; apportioning.
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]
also re-apportionment, "new proportional distribution or arrangement," 1800, American English, from re- + apportionment. Especially in the U.S., "the redrawing of state or federal legislative districts after a census."
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Definitions of apportionment from WordNet
the act of distributing by allotting or apportioning; distribution according to a plan;
the apportionment of seats in the House of Representatives is based on the relative population of each state