The English idiom, “go with the flow,” generally refers to someone letting things happen, being relaxed about random events, or being versatile or able to change plans. This particular phrase has a certain cachet based on the rhyming scheme. It is not uncommon for modern English speakers to use it, usually as a request or suggestion, or, in other cases, as a command. It can also be used to describe someone, who always bows to the majority, or rarely goes against convention.
Word historians date this phrase back to the early 1900s, suggesting that the phrase “go with the flow” was originally based on a metaphor to sea waters. An alternate phrase that used to be used quite often is “go with the tide.” It seems that in modern times, the phrase “go with the flow” is usually preferred over the other. The opposite idea may also be expressed with phrases such as, "going against the flow," or "rowing upstream."
Today’s English speakers have a variety of other phrases that express the same meaning as “go with the flow.” Younger speakers might also tell someone to “hang loose” or “chill out,” both of which are shorter “phrasal verbs,” a linguistic construction that is used quite often, especially in modern English. Also, English speakers might counsel someone to “take it easy.”
In looking at the reasons for the shorter phrases above edging out the use of longer phrases, like “go with the flow,” linguists can see a very definite trend in the English language toward using a single verb to express a multitude of meanings by attaching other single words to it. For example, the use of “chill” as an alternative to the longer phrases has become more and more common. Predictably, adding the prepositional, “out,” has come to be relatively unnecessary, as younger speakers usually understand the verb on its own. In modern slang, some speakers have added the highly idiomatic suffix “-ax” to the word “chill,” resulting in the single word “chillax.” In addition, similar phrases include “be chill,” or “be cool,” or “cool out," some of which directly violate English grammar rules, reflect dialectal usage.
Generally, it seems that with younger English speakers, the association of a calm temperament to cold temperatures has eclipsed the idea of associating these types of demeanors to water metaphors. The water metaphors, such as “go with the flow” have begun to seem overly poetic to many English speakers. These phrases, though, still retain their meanings and are familiar to most native English speakers.