The antonyms for the term "thick fast" vary depending on the context in which the words are used. In terms of viscosity, the antonyms for thick can be thin, liquid, or runny while fast can be slow, gradual, or leisurely. In terms of density, the antonyms for thick can be sparse, scanty, or thin while fast can be sluggish, meandering, or unhurried. The antonyms for thick and fast in reference to traffic can be light, smooth, or flowing while slow, snail-like, or clogged can be used as antonyms for fast when describing traffic. It is important to clarify the context in which antonyms for thick fast are needed for accurate usage.