I’d be sober if gift was a fifth
Because he is in jail he cannot consume alcohol. So if he was presented with a fifth e.g. A fifth of Hennessy, he would still remain sober.
Possibly a reference to pleading the fifth (to avoid self-incrimination). Pleading the fifth may have given Wayne the gift of being out of jail, a gift that he would cherish by pledging to be sober.
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