The term is a modification of the term, lock, which is used to describe a guaranteed winner in sports betting.
The use of this term is introduced by a poster, Richkas, who use the term to gain attention for his sports gambling picks. Due to the fact that his mortal locks are no better than a coin flip, the term is usually used as a comical modifier for sports gambling picks.
There are several variations of the term lock. Some popular variations are, Sicilian Lock, bet your mother's house lock, and $$$mortal lock$$$.
Bet the blues, it is a mortal lock.