a combining form appearing in loanwords from Latin, where it meant “large,” “great”: magnify.
Also especially before a vowel, magn-.
Origin of magni-
< Latin, combining form of magnus
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
word-forming element meaning "great," from Latin magni-, comb. form of magnus "great" (see magnate (n.)).
Online Etymology Dictionary, © 2010 Douglas Harper