That's 略して. The joke is that the portmanteau abbreviation of うんこ and キノコ would still be ウンコ, because キノコ ends with コ as well. How to render that in English is anyone's guess.
!Pteleports said: I don't know how people like Soljashy and Moonspeaker do it.
It is my firm conviction -- supported, I believe, by a preponderance of the evidence -- that they are magic.