An individual can choose to speak in a literal fashion or use creative words and phrases to make his point. For example, a person might opt to use idioms, which are commonly used, non-literal phrases in his speech. "A cold fish" is an idiom that basically means a person lacks emotion and warmth of personality. An individual who is described this way usually gives the impression of being dull, disinterested, and in some cases, rather rude. Sometimes this phrase is even used to describe a person who lacks a healthy sex drive.
There are many different ways a person can describe an individual who seems aloof and lacking in emotion and personality. He could use a literal description or he may choose to use an idiom instead. For example, an individual might use the phrase "a cold fish" in describing a person he believes is unfeeling. Using the idiom doesn't change the overall meaning of the description, but it may make the description seem more interesting to those who are familiar with the phrase. It can also provide a faster way of getting one's point across when compared to the option of delivering a longer, literal description.
Sometimes people find it easier to understand idioms such as "cold fish" when they consider examples of how they might be used. In the case of "cold fish," a person might state that his new coworker is a cold fish in an effort to convey that the coworker doesn't seem friendly or talkative. He could also state that his neighbor is a cold fish because he never has anything interesting to say and always speaks in a disinterested, emotionless manner. In some cases, a person might even state that a romantic interest is a cold fish because she does not seem interested in physical intimacy.
It is important to note that the use of the phrase "cold fish" may not prove appropriate in all situations. For starters, it is usually best applied to situations in which the listener is familiar with the idiom. In the event that the listener lives in a region in which this phrase isn't common, the listener may become confused as he attempts to relate the literal translation of the phrase, a fish that is cold, with the rest of the speaker's statements. Likewise, idioms are generally considered less appropriate for formal speech and writing than they are for causal communications.