Actually, your wrong. "Fuck off" is telling someone something. Dictionary.com is also not a credible source. And slag is considered english, just not proper. Yes the word "dude" and many other are listen in the dictionary.
Fuck comes german word frichen meaning to strike and can describe pain pleasure hate or love
Transital verb:
John Fucked shirly
Intransitive verd:
Shirly fucks
adjective
johns doing all the fucking work:
adverb
Shirly does all the fucking world:
noun
I dont give a fuck
part of word
in fucking credible
Why dont you hear it for yourself
http://www.amishdonkey.com/history-of-the-f-word.php