'onslaught' originates from Middle Dutch, specifically the word 'aenslag,' where 'aen-' meant 'on' and 'slag' meant 'blow or strike.'

'aenslag' transformed into the Dutch word 'aanslag,' and eventually became the modern English word 'onslaught' through Middle English.

Initially, it meant 'a sudden attack or blow,' but over time it evolved into its current meaning of 'a fierce or destructive attack.'