'portentously' originates from Latin, specifically the word 'portentosus,' where 'portentum' meant 'omen' or 'sign.'

'portentosus' changed from the Latin word 'portentosus' to the English adjective 'portentous,' and the adverbial form 'portentously' was formed in modern English.

Initially, it meant 'in a manner relating to omens or signs,' but over time it evolved into its current meaning of 'in a way that is ominous or done with exaggerated seriousness.'