Installment
mid 18th century (denoting the arrangement of payment by instalments): alteration of obsolete estalment (probably by association with installation), from Anglo-Norman French estalement, from Old French estaler ‘to fix’.
wiktionary
A 1732 alteration of estallment, from Anglo-Norman estaler(“fix payments”), from Old French estal(“fixed position”), from Old High German stal(“stall", "standing place”)
From install + -ment, install from Old French installer, from Medieval Latin installare, from Medieval Latin in- and Medieval Latin stallum, stall from a Germanic source (compare Old High German stal)
etymonline
installment (n.1)
"a partial payment on account of debt due," 1776, earlier "the arrangement of payment" (1732), an alteration of Anglo-French estaler "to fix payments," from Old French estal "fixed position, place; stall of a stable, market, or choir," from a Germanic source akin to Old High German stal "standing place" (see stall (n.1)). General sense of "a part of a whole, furnished or produced in advance of the rest" is from 1823. Installment plan is from 1894.
installment (n.2)
also instalment, 1580s, "induction into office, act of installing," from install + -ment.
