Idk stands for I don't know and because the f is next to the d, it is probably an accidental typo.
Light French Kissiing
If you mean slang as in internet slang, then it means "Certified Old Fart".
YAF means in internet slang your a f*ck
LFK stands for light french kissing.
SOS or soz as it is more commonly used is the internet slang for sorry
disconnected
it means *am*
In internet slang it usually means "No strings attached."
MOO is internet slang for "My Own Opinion"
Love And Kisses
Not right now.
NTH stands for nothing in internet slang.