The origin of 'sulk' is obscure, with its first recorded use in English appearing in the late 18th century. It is likely a back-formation from the earlier adjective 'sulky,' which described a person inclined to be sullen or aloof. The word 'sulky' itself may have roots in the obsolete English word 'sulke,' meaning slow or lazy, hinting at a conceptual link between inactivity and a withdrawn, resentful mood. This etymological path illustrates how a term for physical sluggishness evolved to describe a specific, passive-aggressive emotional state characterized by silent resentment and withdrawal.

The word 'sulk' is a simplex word of uncertain origin, possibly related to the obsolete adjective 'sulke' meaning "sluggish" or "idle." Its functional role is as a complete lexical root.