Messenger

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Middle English: from Old Northern French messanger, variant of Old French messager, from Latin missus (see message).

wiktionary

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From Middle English messengere, messingere, messangere, from Old French messanger, a variant of Old French messagier (French messager), equivalent to message +‎ -er. Doublet of messager. Displaced native Old English boda(“messenger, envoy”) and Old English ærendwreca(“messenger, ambassador”).

For the replacement of -ager with -enger, -inger, -anger, compare passenger, harbinger, scavenger, porringer. This development may have been merely the addition of n, or it may have resulted due to contamination from other suffixes such as Middle English -ing and the rare Old French -ange, -enc, -inge, -inghe(“ -ing”) for Old French -age(“ -age”).

etymonline

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messenger (n.)

c. 1200, messager (late 12c. as a surname), "one who bears a message; the bearer of a verbal or written communication," from Old French messagier "messenger, envoy, ambassador," from message (see message (n.)). With unetymological -n- inserted by c. 1300 for no apparent reason except that people liked to say it that way (compare passenger, harbinger, scavenger). From c. 1200 as "a harbinger, forerunner, precursor" (in reference to John the Baptist as the precursor of Christ).