a dead cert

slang Someone who is almost certain to do a certain thing or to succeed in a particular area. Sally is the most qualified candidate by far, so she's a dead cert for the promotion. You're so bright, I bet you're a dead cert for the scholarship. Don't tell Abe, but I heard he's a dead cert for the award!

a dead ˈcert

(informal) a person or thing that is certain to win, succeed, etc: ‘Would you ever bet money on a horse?’ ‘No, not unless it was a dead cert.’

Dead here means complete or total. Cert is a short form of certainty.