Whacking means very big, and it is informal, spoken, and British. It is used to make a size or an amount sound impressive or shocking: "a whacking great hole in the wall", "a whacking bill", "they made a whacking profit". You will very often hear it in the pair whacking great, where great does not mean "excellent" but simply adds to the size — "a whacking great dog" is a huge dog. It can also work as an adverb before another describing word: "a whacking big lorry". The feeling is friendly and a bit exaggerated, the kind of thing said in conversation or in a newspaper headline, never in an essay or a report; in formal writing you would say enormous, huge, or substantial. It is more common with older British speakers, and Americans mostly do not use it this way. Note that whacking is also just the -ing form of the verb whack, meaning to hit something hard — "he was whacking the ball" — so the meaning depends entirely on where the word sits in the sentence.