The idiomatic use of the phrase “banana skin” refers to something negative, something that is either embarrassing or uncomfortable for an individual or party, or something that poses a problem or causes a challenge. It’s interesting to note that in this phrase, which refers to the outer covering of the banana fruit, the alternate term “banana peel” is not used. In general, referring to the skin of the banana as a banana peel is used only when talking about the literal item.
The original use of the phrase “banana skin” is an idiom probably originating from the use of this item in early American comedy, in the beginning of film, when much of the simple humor of cinema was composed of “slapstick” type comedy. In this physical type of comedy, actors often used common props to help create the humorous scenes related to their physical activity. One of these was the banana peel or skin, where an individual would frequently slip on the banana skin and fall comically to the ground. Over time, the banana skin came to be associated with general liability, i.e. people slipping on things.
In more modern uses of the phrase, people might talk about a banana skin as not just a humiliating or embarrassing mistake, but something that has serious consequences for an individual or a group. For example, in business, leaders might talk about an unanticipated challenge like a legal liability as a banana skin, not just in terms of the negative publicity that it may cause for the business, but also in terms of its economic effects. The primary use of the term, though, still implies some kind of compromised position, an embarrassment, or a loss of reputation, much like the idiomatic use of “egg on (his or her) face,” where it’s indicated that the subject has suffered in terms of image or reputation.
Those who combine the study of idiomatic language with actual sciences point out that the use of the idiom banana skin connects the actual item to a negative metaphor. In fact, there is a lot of nutrition in the banana peel or skin, which is not often consumed in many English-speaking countries. More often, the banana peel or skin is simply seen as a waste, or, in the above idiomatic phrase, something even more negative, a liability or an obstacle.