This is a old used English northern coal miner slag: somebody that is so loose they needed handles I.E straps on their back to hold on to during coitus. ( more a homophobic insult than a heterosexual insult)
Defined this means "fuck off tha strap back" "tha's nowt but a strap back" "tha's less use than a strapback"
The unfortunate circumstance that occurs when a male is receiving oral sex and the person giving the oral sex accidentally applies teeth to the man's penis. Typically, this occurs when the penis is exiting the giver's mouth, hence 'back' scrape.
when you're doing anal and you're getting so into it doggie style and you scrape your partner's back but pull out at the last second and cum in the scrapes.