Formal:
* Absurdity
* Frivolity
* Inanity
* Rubbish
* Trash
* Trivial
* Utterance (when referring to a nonsensical statement)
Informal:
* Balderdash
* Bosh
* Bunk
* Gibberish
* Hogwash
* Hot air
* Poppycock
* Rot
* Stuff and nonsense
The best synonym to use depends on the context and the desired level of formality.