Learn how to say Onslaught correctly in English with this tutorial pronunciation video.
Oxford dictionary definition of the word onslaught:
a fierce or destructive attack:
a series of onslaughts on the citadel
an overwhelmingly large number of people or things:
in some parks the onslaught of cars and people far exceeds capacity
early 17th century (also in the form anslaight): from Middle Dutch aenslag, from aen ‘on’ + slag ‘blow’. The change in the ending was due to association with (now obsolete) slaught ‘slaughter’