crushing blow; heavy blow; hard blow
Figurative noun for dealing a decisive, damaging strike to a person, organization, or plan. Often used with verbs like 食らわす or 与える.
The scandal dealt a crushing blow to the administration.
The new regulations delivered a heavy blow to the industry.
大打撃 is a more common and neutral term for a major blow or damage, while 一大鉄槌 carries a stronger, more dramatic nuance of a decisive, hammer-like strike.
Compound of 一大 (one big, a great) and 鉄槌 (iron hammer), literally 'one great iron hammer', used figuratively for a crushing blow.