From milli- + milli-. <span class="searchmatch">millimilli</span>- (obsolete) micro- (in the names of SI units)...
From <span class="searchmatch">millimilli</span>- + gram / milli- + milligram. millimilligram (plural millimilligrams) (rare and nonstandard) 0.000001 grams, as a unit of mass. microgram...
millimicro- millimicroampere millimicromole millimicron millimicrosecond <span class="searchmatch">millimilli</span>- millimilligram millimolar millimole millinewton millinormal millioctave...