basically, hysterically crying. or just crying really. doesn't have to be hysteric.
joe: didn't hand my biology homework in. davies gave me a detention so my dad said i can't go out with you guys tonight.
jack: ha! sheddin mate.
hannah's boyfriend dumped her by leaving a sticky note on her front door. she was proper sheddin.
Reffering specifically to musicians
but can be used for any subject
Sheddin' is used to describe an evolution of skill or to practice to another level of skill
I was sheddin' my guitar all night to learn that new riff of that one song
"Sheddin" a term that is commonly used among musicians that in some circles may be referred to as a "jam-session". Sheddin is underground music jargon that artists use to reference a gathering of musicians.
Jake: "yo man you got your guitar? Lets go sheddin'!"