The characters combine literally to mean “foot steps on two (measure word) boats.” The meaning is instantly understandable from this literal image: an act of foolishness and instability that cannot be maintained.
The idiom 脚踏两只船 reflects a strong cultural emphasis on loyalty (忠诚, zhōngchéng) and single-mindedness (专一, zhuānyī) in Chinese society, particularly in personal and professional relationships. The act of straddling two boats is seen not just as indecisive but as inherently opportunistic and dishonest. It implies that the person is not genuinely committed to either side but is simply trying to maximize their own gain, a behavior that is generally looked down upon. A close Western concept is “sitting on the fence,” but there's a key difference. “Sitting on the fence” often implies passive indecision or neutrality. 脚踏两只船, however, usually implies active engagement with both sides for personal benefit. It carries a much stronger moral judgment. In romance, it's less like “I can't decide who I like more” and more like “I'm actively dating two people behind their backs.” It is the direct cultural equivalent of “two-timing” or “playing both sides,” and it almost always ends with the person “falling into the water”—facing negative consequences for their duplicity.
This is a very common idiom used in everyday conversation. Its connotation is almost always negative and critical.