Basically, it's a somewhat crude synonym for "bellyaching," which is a colloquial term for "complaining," as the previous answerer says.

However, when used as an adjective, it's a somewhat crude synonym for "cool," which is a colloquial term for "admirable," "attractive," or "desirable"-- as in the the lyrics to the Dead Milkmen's song "Bitchin' Camaro" (1985):

Bitchin' Camaro, bitchin' Camaro/ I ran over my neighbors/ Bitchin' Camaro, bitchin' Camaro/ Now it's in all the papers

et cetera...

I recall a much older instance of the adjective use of "bitching" in the Dead Milkmen sense--from Jim Brosnan's 1962 book Pennant Race, where he quotes Pittsburgh Pirates first baseman Dick Stuart as saying [inexact quote here because I don't have the book with me, but I'll try to remember to look it up when I get home], "I add 15 points to my batting average just because I know I look bitching up there [at the plate]."

Basically, it's a somewhat crude synonym for "bellyaching," which is a colloquial term for "complaining," as the previous answerer says.

However, when used as an adjective, it's a somewhat crude synonym for "cool," which is a colloquial term for "admirable," "attractive," or "desirable"-- as in the the lyrics to the Dead Milkmen's song "Bitchin' Camaro" (1985):

Bitchin' Camaro, bitchin' Camaro/ I ran over my neighbors/ Bitchin' Camaro, bitchin' Camaro/ Now it's in all the papers

et cetera...

I recall a much older instance of the adjective use of "bitching" in the Dead Milkmen sense--from Jim Brosnan's 1962 book Pennant Race, where he quotes Pittsburgh Pirates first baseman Dick Stuart as saying [inexact quote here because I don't have the book with me, but I'll try to remember to look it up when I get home], "I add 15 points to my batting average just knowing I look bitching up there [at the plate]."