Slang term of maggot. Used to describe someone who is 'pissed as fuck' or strongly under the influence of drugs
"On the weekend me and Johnno got like hell magsy it was sickkkk"
"Truuuuuuuuuuueeeeeeeeee lets get helllll magsy"
"Fuck i'm soo magsy right now i funnelled like 23 coldies"
"Shitttttt i just had like 4 cones im fucken maggsy as shit"