The phrase "bob down" is commonly used to describe a quick, upward and downward movement of the head or body. Synonyms for this phrase include: dip, duck, nod, bounce, jerk, twitch, and bobble. Each of these words carries a slightly different connotation, and can be used in various contexts. For example, "dip" may describe a subtle movement of the head, while "bounce" suggests a more exaggerated motion. "Jerk" and "twitch" have a more abrupt feel, while "bobble" implies a lack of control or stability. Overall, using synonyms for "bob down" can add nuance and variety to your writing or speech.