mh – mental health
ph – physical healh
pos (or +) – positive
neg (or -) – negative
~ (sometimes ?) – unsure if it’s positive or negative, or somewhere in-between
ment – mention
pol – politics
uspol/ilpol/ukpos/eupol etcetera – politics, but region specific, the first two letters are an indicator (uspol – united states politics, eupol – european politics, and so forth
oh – overheard
ooc – out of context
nsfw – not safe for work
tmi – too much information
sa – sexual assault
csa – child sexual abuse
sh – self harm
sui – suicide
sfw – safe for work
cw – content warning
cn – content note, haven’t seen this one personally, but used for context rather than warnings
alt – referring to alt-text, which are media descriptions for the visually or aurally impaired
ec – eye contact, usually an image of someone or something looking directly at the camera
what – commonly it’s used for “hotdogs are tacos” type deals, stuff that seems silly that’s meant for messing with people
??? – confusing or nonsensical
veg – usually vegan, sometimes vegetarian
exp – experience
qt – quote toot/tweet
rt – re(toot/tweet)
qn – quote note
rn – renote
qp – quote post
rp – oddly, usually roleplay, but it could be repost too
lb – last boost
ref – reference

– tone indicators –
/lh (or just lh) – lighthearted
/j (or just j) – joking
/s (or just s) – sarcasm
/srs (or just srs) – serious
/sx, /x (or just sx or x) – sexual
/nx, nsx (or just nsx or nx) – non-sexual/not sexual
/ly (or just ly) – lyrics
/r – romance
/gen (or just gen) – genuine

– queer community abbreviations –
enby/nb – a non-binary individual, someone who is neither boy nor girl
terf – trans exclusitory radical feminist, someone attempting to fight for women’s rights while only fighting for some of them
fart – fascist aligned radical transphobe, used for the same group as terf
transmed – someone who believes trans people can only be boy or girl, must medically transition, and must have dyshporia in order to be trans (i.e. a malevolent wilfull ignoramus)
lgbtqia+ – lesbian gay bisexual trans queer intersex asexual etc., used as a general term for the queer community
alphabet mafia – a slang term for the queer community making fun of the lgbtqia+ acronym
grsm – gender romantic and sexual minorities, a more inclusive umbrella acronym for the queer community
hrt – hormone replacement therapy, a medical process using supplements to artificially increase or decrease the hormone levels of an individual, resulting in gender affirming physical changes
ts – trans
saph/sapphic – in reference to the historical lesbian poet Sappho, who was a significant face of early lesbian communities, sometimes used in place of the term lesbian

Feel free to reply with more and I’ll edit them in
Link to this list on my website for longevity