Keep the devil off my heels
But please keep me on my toes
Continuing to use the comparison of different parts of the foot, to keep someone off your heals means to keep them away from you.
To stay on your toes means to stay aware and to not fall, and not give up.
Lupe is asking the Lord to keep the Devil as far away from him as possible, but also asking to give Lupe the strength to be aware of the Devil and deal with the presence of him just in case.
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