serious business

Translations
серьезное дело
Starting a new company is serious business.
Начать новую компанию - это серьезное дело.
Definitions
A phrase used to describe a situation or activity that is important, significant, or requires careful consideration and attention, often implying that it should not be taken lightly or treated as a joke.
Words
serious

adjective
серьезный
business

noun
бизнес