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закадычный друг stresses
gen.	crony; special friend; sworn friend; bosom-friend; intimate friend (sashkomeister); soul mate (molten); a thick friend; mate (Виталик-Киев); fast friend (Баян); bosom chum (Mr. Lite); intimate; familiar; a near friend; a peculiar friend; shoulder friend (человек, на плечо которого всегда можно опереться, поведать ему все свои горести, "поплакаться в жилетку" LisLoki)
Игорь Миг	best mate
amer.	kemosabe (Val_Ships)
amer., inf.	cousin
book.	boon companion (literary) a very close friend (Longman Dictionary of Contemporary English Sonora)
brit.	chum
crim.jarg.	yak; yock
inf.	bosom buddy (Andrey Truhachev); brother from another mother (Tion); bosom pal (dkozyr)
Makarov.	bosom friend
slang	solid friend; ace boom-boom (Svetlana D); ace boon-coon (Svetlana D)
закадычные друзья
gen.	swore friends; near friends; bosom friends; sworn friends; as thick as thieves; intimate friends (sashkomeister)
Игорь Миг	BBFs
idiom.	thick as thieves (VLZ_58)
slang	besties (almost always refers to girls Liv Bliss)

закадычный друг: 22 phrases in 5 subjects

American usage, not spelling	1
General	9
Informal	1
Makarov	10
Vulgar	1