What does DERP mean?
A derp is a dumbass and a derpina is a girl dumbass.
Other definitions of DERP:
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How to use the term DERP:
what a derp, he just ran over the sidewalk
More slang terms:

APPT
appointment

HBD
Happy birthday

NGL
Not Gonna Lie

LOLCI
laughing out loud, crying inside