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.

Slang

aligatori

to speak a national language different from one's own at an Esperanto congress.

bonantagulo 

a lazy person who learns only a few simple Esperanto phrases such as "Bonan tagon!"

dando 

fashionista, someone who is involved in fashion trends

etoso 

ethos, atmosphere, "karma" (as of an Esperanto congress, etc.)

giko 

geek

gufujo 

a "chill room," a place or event that Esperantists gather at to relax and/or decompress

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"

nerdo 

nerd

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

verdulo 

a proud/fervent Esperantist, literally "green person."

volapukaĵo 

nonsense (after the obsolete constructed language Volapük)