When a student of special needs brings a thermos with him to lunch. After eating the contents of the thermos, he proceeds to the restroom where he has dishwater (bad diarrhea) back into the thermos. (A funnel may be used at times dependent upon how violent the dishwater is) The thermos is later brought home to the mother where she is greeted by her sweet little boy/girl, who screams "surprise" when the thermos is opened and contents are discovered. (contents are still warm) Pissed off, the mother does a quick rinse of the thermos and repacks the same lunch in the same thermos for her special child the next day.
Now, here is where the definition gets tricky: Thermos Mermos can be used as both a verb or noun.
Utilization as a verb states that Thermos Mermos is the act of refilling a thermos with excrement (must be diarrhea) so it's still warm when brought home to mother. Note that screaming "surprise" when the mudpies are found is and added touch to Thermos Mermos and does not have to be stated in all Thermos Mermos accounts.
Usage as a noun (Person): Thermos Mermos could refer to both the "actor" (person who fills the thermos with the shit-funnel) or the finder of the excrement inside the thermos.
Usage as a noun (Thing): A thermos with volcanic shit bubbling inside.
Brian: Dude, I've gotta go drop the Cosby kids off.
Nate: Why are you bringing your thermos to the bathroom?
Brian: I'm gonna Mermos this Thermos.
Nate: THERMOS MERMOS!