One very long word that starts with m is: microcryptocrystalline (22 letters)
This is the longest word in the whole world. It has 189,819 letters on it.
methionylglutaminylarginyltyrosylglutamylseryllleucylpheny
A four letter word that starts with J and ends with M: jism.
A seven (7) letter word that starts with M and ends with O: memento
Mussel
mess
methionylglutaminylarginyltyrosylglutamylseryllleucylpheny
Make is a word that means to create and starts with "m".
A four letter word that starts with J and ends with M: jism.
A muscle is a 7 letter word that starts with m and helps you run fast.
A seven (7) letter word that starts with M and ends with O: memento
mademalemanemaskmakemashmathmartmarkmessmeltmeekmeanmealmaremintminemindmilemistmastminkmitemaltmatemoatmissmoonmoodmashmostmolemoltmoremeremeatmeetmustmuskmusemythmothmushmuchmovemoanmademiremacemainmane
MeatMateMakeMindMineMintMoodMeekMaleMailMassmast
mulch
mend
measurement
morons.
magmamajormakermangomangymanormarrymaybemauvemeantmetermessymeritmerrymilkymightminormintyminusmirthmistymitermincemoistmoldymonthmoneymoodymousemoosemotormountmountmouthmossymottomoviemuddymurkymuskymustymutedmazed