This is a common acronym used in the Lego community - what does MOC mean?
MOC stands for "My Own Creation".
My Own Creation. Any LEGO creation designed and built by a LEGO fan without instructions. Generally pronounced “mock.”*
Found at this Lego glossary.
Brickipedia's version from their own glossary:
MOC - My Own Creation, a fan- made LEGO model
*A side note: There may be some LEGO fans who pronounce "MOC" the way the individual letters are pronounced - effectively saying "Em-Oh-See".
A MOC or a Lego MOC, are the initials of My Own Creation, so, if you have made a creation or a Lego model out of your mind, I mean, with no instructions, you've made a MOC.