an unexpressed doubt or qualification about a situation, person, etc.
Origin of mental reservation
First recorded in 1600–10
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a tacit withholding of full assent or an unexpressed qualification made when one is taking an oath, making a statement, etc
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