mass in

verb

(adverb) to fill or block in (the areas of unified colour, shade, etc) in a painting or drawing

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