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серьёзное дело stresses
gen.	hard ball; hardball
austral., slang	a big deal
busin.	serious business (Alex_Odeychuk)
dipl.	grave matter
inf.	a big deal (Ivan Pisarev); big deal (Ivan Pisarev)
серьёзные дела
gen.	key matters (bigmaxus)

серьёзное дело: 35 phrases in 7 subjects

Australian	1
General	14
Idiomatic	2
Jargon	1
Law	6
Makarov	6
Slang	5