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2.
The act of holding in faecal matter beyond it's natural release point, to the point where it is bubbling to get out. At this point the dirty payload is then released onto the waiting stomach of a loved one in an act of tarmacking (see 'tarmacking')
Lee has had one simmering on his hob for a few hours now while Laura is waiting by the fire for a good tarmacking.
by Crebs January 15, 2010
 
1.
The act of leaving someone's friend request on Facebook unanswered: i.e. neither accepting nor rejecting.
"She added me on Facebook, and I'm just letting her simmer there"

"Her friend request is simmering"

"Why have you just left me simmering on Facebook???"
by CJJC March 03, 2013