with the flat side, rather than the edge, foremost or in contact.
Also flat·ways [flat-weyz] /ˈflætˌweɪz/.
Origin of flatwise
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
with the flat or broad side down or in contact with another surface
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