A very special, good friend that's been in your life for years, typically since youth. From the Johnny Mercer and Henry Mancini song, "Moon River", which was featured in the classic movie "Breakfast at Tiffany's".
There are your good friends: people who love you. And then there are your huckleberry friends: people who've known you for years and have stuck by you and love you no matter what.
"We're not serious...we're just huckleberry friends."
"We're after the same rainbow's end, my huckleberry friend, moonriver and me."