About
"Trap" is an internet slang term used to refer to a fictional character whose outward appearance is inconsistent with one's perceived gender.
I don't recall seeing anywhere where felix was explicitly stated to be trans though, just as being biologically male, so likely there isn't a canon gender identity for him (i suppose when the anime adapts that part of the source material it might change, also idk if japan actually has the same concept of gender and sex being often considered seperate, and recent developments might have changed or complicated things).
Could almost be mistaken for a woman until you are in the bedroom with one. In order to create a safer world for the trans community, work needs to be done from the ground up. To men controlled by toxic societal norms, trans women are just deceitful crossdressers. In extreme cases, these thoughts eventually escalate to violence or murder.
Those who use it typically have a poor understanding of its far-reaching implications, or even what it means. It justifies violence that stems from insecurity. However, the word has drawn criticism and stigma for its derogatory connotation against the transgender community.
Origin
The term "trap," not to be confused with "trap house" (a place where illegal drugs are bought, sold and used) or trap music in hip hop culture, stems from a reaction image based on a scene from Star Wars: Return of the Jedi, wherein Admiral Ackbar is quoted as saying "IT'S A TRAP" upon discovering an ambush by an enemy fleet.
As the reaction image continued to gain traction across anime hub sites like Something Awful and 4chan over the next decade, the word "trap" itself gained recognition as a slang label for a transgender person in general.
Spread
Beginning in 2004, the Admiral Ackbar quote spawned hundreds of parody sites featuring the soundbite of Admiral Ackbar’s quote on YTMND, with the most notable instance uploaded by user MultiKoopa on June 2nd, 2005, which has garnered more than 313,000 views (as of April 2016).
Around the same time, shortly after the launch of the imageboard community 4chan, the phrase became closely associated with traps, a slang term for a photograph of a model or illustration of an anime character wherein the gender of the subject seems to be ambiguous or androgynous in appearance.
4chan users, in addition to creating the internet theme day "Trap Thursday," described androgynous anime characters as "traps," a long tradition within anime and manga.
That however was proved wrong when that history was wrote, because Felix was the one that asked for it, not otherwise.
Also, when asked if Ferris could ever go back to be Felix the Author said no.