2. Grandiloquent, pompous speech or writing.
3. Language that is intentionally difficult, usually to make something sound more important than it really is.
4. Inflated or turgid language; high-sounding language on a trivial or commonplace subject; "fustian"; "tall talk".
5. Cotton or other material used to stuff garments; padding.
A language that is full of long or pretentious words, used to impress others; nonsense; pretentious talk; and bold and deceitful absurdities. Slang descriptions include: "baloney, hot air".
Bombast has had a confusing development
The term originally referred to cotton padding (from Greek to Latin to French [cotton stuffing]); it now means, swollen language or high-flown diction that exaggerates the importance of the subject.
The original meaning (now obsolete) is "cotton or any soft fibrous material used as padding or stuffing". It is derived, through Middle French bombace from Medieval Latin bombax, which means "cotton", in spite of the fact that the original Latin bombyx and its Greek source refer to silk.
According to one source, the shift was occasioned by an error going back to the Roman scholar Pliny, who had reported that cotton was produced by an insect analogous to the silkworm.
Bombast has been retained in modern English because it took on the figurative sense of "a pretentious or inflated style of speech or writing". So the basic sense of "stuffing or padding" has survived, but now the stuffing consists of words rather than cotton.