A sarcastic remark is one that is sharply ironic or sneering, often intended for humorous effect. Sarcasm is often conveyed through vocal inflection, and may be used in an indirect manner, as in “What a fine musician you turned out to be!” or in the form of a direct statement, “You couldn't play one piece correctly if you had two assistants.” The adjective caustic is defined as “severely critical” or (lo and behold!) “sarcastic.” To understand how it’s different from sarcastic, it’s helpful to look at the original meaning of caustic, “capable of burning, corroding, or destroying living tissue.” A caustic agent, such as lye (which is also called caustic soda), is one that can cause tissue damage. The destructive force of this primary sense is carried into figurative uses of the term: a caustic remark is one that can wound or corrode—even in service of a joke.

To refrain from something is to abstain from an impulse to say or do that thing. The idea of (self-) restraint is reflected in the word’s origins: refrain comes from the Latin verb refrēnāre meaning “to bridle,” or to control or hold back a horse. To forgo something is to give it up or to do without it; a patient might forgo medical treatment or an executive might forgo a bonus. Beware that forgo is sometimes spelled forego, but forego has a meaning all its own: “to go before” or “to precede.” To avoid ambiguity (and to please the editors in your life), forgo the e when spelling this subdued synonym.

To indulge in something, such as a decadent dessert, is to yield to a desire for that thing, or to allow oneself to follow one’s will. This verb is commonly used of things one typically limits or avoids, and so it is bound up with the idea of giving oneself permission to do something or giving in to something that is usually off-limits. To luxuriate in something is to revel in it, or to enjoy oneself without limitation or restraint. Luxuriate implies a more delighted and celebratory kind of partaking. This sumptuous synonym may call to mind extravagance (what we associate with the word luxury), but one can luxuriate in pleasures big or small.