What is got over the Devil’s back (that’s by knavery), is spent under the belly (that’s by lechery).

Middleton’s aphorism links two vices as cause and consequence: wealth acquired “over the Devil’s back” (through fraud, cheating, or other knavery) is predictably dissipated “under the belly” (through lust, sensuality, and self-indulgence). The image suggests a moral economy in which ill-gotten gains do not stabilize a life but feed appetites that accelerate ruin. It also implies a satiric social observation: the same characters who are unscrupulous in getting money are often reckless in spending it, especially on sexual pleasure. The line’s blunt bodily metaphor is characteristic of early modern city comedy’s exposure of hypocrisy and vice.