TTFN — is an initialism for a colloquial valediction, 'ta ta for now', based on 'ta ta', an informal 'goodbye', approximately equivalent to 'bye bye', 'see ya' or 'laters'. The expression came to prominence, in the UK, during the Second World War. It has currently become the fifth most commonly recognized initialism in English modern diction, behind ROFL*, OMG*, and WTF*.
TTFN is still used occasionally in ham radio as an ending to a transmission but was brought into popular use in the UK in 1941 in the weekly radio comedy ITMA by the character Mrs Mopp.
'Ta ta for now' caught on with the British public so much that it was often uttered by dying people as their last words.
ROFL — Rolling On the Floor, Laughing; used to indicate great amusement at something in a discussion group, etc. (Internet slang)
OMG — Oh my God; its first known attestation is in a letter from John Fisher to Winston Churchill, dated 1917, as «O.M.G.».
WTF — What the fuck (military slang, Internet slang, text messaging)