To have performed sexually at such a profound level that the utilised orifice has caught fire due to friction.
This could be down to a number of factors:
1. Overly fricative appendage surfaces, e.g. a scaley
penis
2. Use of a lube with an insufficient smoke
point for the activity, e.g. high-velocity anal sex lubricated with Butter (176°
C) rather than Ghee (250°C) that meets an emulsifier, e.g. 'Baby
Gravy', thus overcoming a natural fire retardant, e.g. 'Fannybatter'
Example 1 is
pure friction but usually occurs before Cum enters the picture so is not relevant to this analysis
Example 2 is the
true Cumbust in that it is down to the complex chemical interations of these substances when extreme kinetic energy is applied to them
Bob - Jesus, did you see what happened when Bill banged Brenda's Bum hole?
Babs - Nope, wtf?
Bob - She Cumbusted, flew into low earth orbit, left a trail of winnets in the night sky that looked like a meteor shower
Babs -
Shit, talk about blasted into maternity by a guided muscle....