MOST dual-purpose words, such as wicked, depend for their meaning on who happens to be using them: bad when spoken by parsons, good when spoken by pagan teenagers, say. is different. It more often does a quick-change act by going into another part of speech. Ken Livingstone used the adjective to some effect in the House when he accused Bernard Manning, the bad-taste comedian, of having been offensive about black people (he had called them niggers). Use it as a noun and there is no reproach, rather the reverse. A war leader who fails to mount one could be ready for his pension.
It wasn't till the 18th century that people made a noun of it and began talking about being "on the offensive". The adjective came first, having been taken from defendere, to strike, which didn't necessarily imply aggression (defendere was also the word Romans used when they stubbed a toe). But then it rapidly got a metaphorical meaning too, as it had in Latin, and became more and more disapproving.
Anyway, for some 150 years the adjective had always done for either meaning - the neutral one, which was simply the opposite of defensive, and the one we now find it so easy to associate with Bernard Manning; and it can still be ambiguous. What, for example, is meant when someone mentions an offensive footballer? Is he a good striker, or has he been rude to the referee again (both, perhaps)?
In his reply to Ken Livingstone, John Major preferred not to use the word, observing instead: "I certainly think that everybody should avoid expressions that give offence to those who are on the receiving end." This was rather craven of him. Being offensive is a great deal more unpleasant than merely giving offence.