It refers to [XD], which is supposed to be a new [lol], but it is a lot funnier. It means: X(Fucking)D.
> Bob tripped up, and landed in a pile of cow manure face first.
> Me: XFD

学英语单词
a-thrill
according to holye
acetylation number
Aiema
alumina-silicate ceramic fibre
backwashed
Baishan
better-balanced
bistable generator
Bretton Woods
Brixen
bulse
bus drivers
capacity of oilwell
catechizeth
cit (conductivity indicator/transmitter)
concave shapes
contiguous receptor
critical plasmolytic concentration
cryptophony
cubic capacity of permanent bunkers
Diapheromera femorata
Dioctophymata
direct-to-home satellite
disclaimer of opinion
disowning of offspring
electric putty
eucarpic
extrinsic acceptance
fast hemoglobin
floating aerator
foig
full reflection
gadded
gallenbach
Garrett County
gel paint
give someone a wet shirt
governor lever
hairflair
hand sth out
hanging about
heat exchanger leak detection probe
herald-journal
humeroradial
ingulf
insatiableness
interested
Isometrus
kelvin's model of viscoelasticity
lindsaea commixta
line band
litchi nuts
local mass eccentricity
logn feed
Marchantia polymorpha
marie louise elisabeth vigee-lebruns
medical charges
mepacrini hydrochloridum
metchlorocrurins
michelis
mina cloth
minimum crossing altitude
moorstone
multiplex radiotelegraphy
Mustard operation
network modem
nonmagnetic sheathed cable
nuclear radiation decomposition
observed temperature
outs-of-sight
over-soon
paddy field boat
Perlia's nuclei
pittenger
prairie lotus
Proheptatriene
pure component
Pyaozerskiy
reyerite
sered
stagnatory
stand comparison with
stator
statospermus
stencilization
stoping
structural ambiguity
Sunchon
supermaxilla
superplants
taiaiake
take matters into one's own hands
target scintillation
timecard
type c display
ungreeting
visual match
vitamin(e)
wicketless
xii-xiv
ylang-ylang ylene