In a most particular sense, bombast (the noun) and the resulting adjective, bombastic, refer to pompous and grandiloquent use of language.
Its meaning has
expanded to include people being pretentious, flamboyant or boastful in social
contexts.
The history of word has little to do with its current use. The
now obsolete meaning refers to material used for padding, from the Medieval
‘bombax’ – cotton.
In this context, look at bombacaceous – a family
of tropical trees eg kapok (from which ancient tribes sourced a type of cotton
– West Africa, South East Asia and Indonesia) and baobab with very thick stems
that have water-storage capacity.
The adjective bombastic arises from the
same source…. Just love the picture – a person as full of words as the baobab
tree is full to bursting with water!