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2.
Two female who have became best of friends. Best female friend.
This term was used back in the 80's to discribe friendship between two women. But now in the 21st century its replaced with Best Friend Forever (BFF) or Best Friend (BF).
I can't wait to bosom to see my bosom buddie, we were so close as youngins.

Me and her were bosom buddies in high school.
My bosom buddy is getting married.
by Sherekay October 10, 2010
 
1.
True friends who know and understand one another, even despite occasional fits of hormonal rage.

See also deep dawg.
Nathan and Clare were bosom buddies, even during their synchronised hormone fluctuation cycles.
by lols-royce May 24, 2010
 
3.
When two friends get together for casual sex
casey x cunt : bosom buddies ?
savannah x suicide : only if johnny x jopito can join us. ;]
by DAVE BAKER June 03, 2008