The phrase "serious blow" means a significant and detrimental impact or setback. It implies a major negative consequence or damage to a person, organization, or situation. Full definition
These were serious blows for a team which prides itself on defensive solidity; a team, indeed, whose very survival was dependent on defensive solidity... (espnfc.us)
Real Madrid have been taking serious blows in the last few weeks. (caughtoffside.com)
Deconstruction's radical hermeneutics strikes serious blows at certain traditional theories of meaning by showing how unstable an affair language is. (religion-online.org)