OP is original poster - the person who originally started the thread.
Other examples - MIL (mother-in-law), FIL (father-in-law), DIL (daughter-in-law), LTR (long-term relationship), GF (girlfriend/grandfather), BF (boyfriend). SIL can also mean son-in-law, depending on the situation.
Girls grammars in the 60s (and beyond)
Favourite small treat when feeling blue or conversely a bit flush
) if I did.