- variant of in-2 (by assimilation) before l: illation.
- variant of in-3 (by assimilation) before l: illogical.
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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
Word Origin and History for il-
assimilated form of Latin prefix in- used with words beginning in l-; see in-.
Online Etymology Dictionary, © 2010 Douglas Harper