There are very few idiomatic or slang words in Esperanto, as these forms of speech tend to make international communication difficult—working against Esperanto's main goal. The suffix -ĉjo may be used to describe something in an affectionate manner, but this was defined ever since the first Esperanto dictionary and should not be considered slang.
bonantagulo = a lazy person who learns only a few simple Esperanto phrases such as "Bonan tagon!"
etoso = ethos, atmosphere, "karma" (as of an Esperanto congress, etc.)
kabei = to disappear from the Esperanto movement (named after author Kabe)
krokodili = to speak one's native language among a group of Esperanto speakers
mojosa = "cool"
samideano = Esperantist. Originally, someone "who has the same idea" as another.
verda papo = "green pope"; one who talks at great length about the ideals of Esperanto
volapukaĵo = nonsense (after the obsolete constructed language Volapük)