lower than a shoelace. أدنأ من الشسع

This is from the root meaning baseness, if they pronounce it with a hamza. If they omit the hamza, they say: closer to a person than his shoelace, meaning something very close to him.
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If the intended meaning is “lower than a shoelace” (with a hamza), meaning the most despicable and vile thing imaginable, there is no common literal equivalent in English. However, the closest proverbs and expressions are:
As low as they come.
Extremely vile and base.
Lower than a snake’s belly. (Colloquial) The lowest of the low.
A low-down person.
So, the proverb can be translated according to its meaning as:
He is as low as they come.
However, if the intended meaning is “closer to a person than their own shoelace” (without a hamza), meaning very near, the closest English equivalent is:
Closer than your own shoelaces. (Explanatory formulation)
Or:
Close at hand.
Right under your nose.
Therefore, based on the explanation I have given, the best equivalent for “lower than a shoelace” is:
As low as they come.
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من الدناءة هذا إذا همزوه فإذا تركوا الهمز يقولون : أدنى إلى المرء من شسعه للشيء القريب منه جدا