Which term describes something that is sly and intended to deceive?

The term that describes something sly and intended to deceive is "insidious." This word refers to actions or strategies that appear benign or subtle at first but ultimately lead to harmful consequences. In the context of deceit, something insidious often operates in an underhanded manner, gradually undermining trust or causing harm without being immediately obvious. This aligns perfectly with the idea of slyness, as true intentions may be hidden under a facade.

The other terms do not convey the same meaning. For instance, "stolid" describes someone who is emotionally unmoved or indifferent, which does not relate to slyness or deception. "Opulent" refers to richness or luxuriousness, and has no connotation of deceit or slyness. "Pliable" denotes flexibility, whether physically or in terms of ideas and opinions, which does not connect to the notion of cunning or insidious behavior. Thus, the choice of "insidious" is clearly the most accurate in this context.