a combining form meaning “sleep”, used in the formation of compound words: somnifacient.
Also especially before a vowel, somn-.
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before a vowel somn-
Word Origin for somni-
from Latin somnus
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
before vowels somn-, word-forming element meaning "sleep," from comb. form of Latin somnus (see Somnus).
Online Etymology Dictionary, © 2010 Douglas Harper
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.