Why the opposite meanings of the word “bollocks”?

As the other posters have stated these type of two-faced word usages are very common in vernacular speech.

Over the last few decades, more and more have crept in to the common tongue. Usually bringing a traditionally negative word in to a positive usage. Using a word as if it was its own antonym. Presumably it stems from the fact that what is bad often invites temptation and what is tempting must be desirable and what is desirable must be good...

• wicked - that girl is wicked; what a wicked idea (traditional negative, modern postive)
• sick - that band was sick (traditional negative, modern positive)
• wild - that land is wild; what a wild ride (traditional negative, modern positive - to thrill seekers)
• bad - see Michael Jackson (open to debate given)
• bollocks - what a load of old bollocks; that was the bollocks [note old vs the as the adjective, one (implied to be) negative old, the other the as a superlative]
• etc.

There were a lot of phrases created in the early part of the last century that were very much playing with language to create a ridiculous phrase meaning something that was good - that' gives us "the bees' knees", "the cat's whiskers" and similar things. "The dogs bollocks" is a modern take on that. If I was to guess at a history I would think it was an anglicisation of "the mutt's nuts" which I would expect to date to a similar time to the other phrases I mentioned. It has probably become popular mostly because there is something inherently funny about the word "bollocks."

I've never heard either "the bees bollocks" or "golden bollocks" used by anyone, but I guess if they are they are similar to the above.

However "it's bollocks" is derogatory in the same way as "it's arse", "it's pants" or anything similar. You can pretty much put anything vaguely taboo after "it's ..." and you have a convenient playground-appropriate derogatory term. Bollocks does well mostly because it's quite funny and very satisfying to say.

I guess any confusion would be because you were looking at the word as a token of meaning, rather than the phrase and context, which ends up being most of language. Sometimes these antipathetic contexts might collide- in the late eighties I might have had a conversation with my granny in which she said the Poll Tax was wicked and I said the new Salt'n'Pepa record was wicked and someone reading what we said might not realise we mean't exactly the opposite. Come to think of it, maybe I wouldn't have either. I might well have thought my granny thought the Poll Tax was really excellent, like the new Salt'n'Pepa record.

I don't know about the phrases in question, but words that have variant meanings that are complete opposites are not uncommon in English.

cleave

• split or sever(something), esp. along a natural line or grain
• stick fast to or adhere strongly to (something)

fast

• moving or capable of moving at high speed
• immobile or hard to move; firmly or securely held

I feel such things are simply part of the endlessly entertaining panoply of the language we call English.