意思是现成的、即买即用的，且暗示不是定制化的。

If a product is off the shelf, it can be used straightaway without any setting-up.

如果要完全理解这个词的来历，可以用off the shelf that you can use it immediately. 省略了后面的话。

类似的还有off-the-peg （现成的（衣服））

ps：注意使用时中间加横线连接，表示一种成语和习语。

需要注意  on the shelf指的是被搁置，被遗忘

On the shelf
As meaning without prospects of marriage, usually applied to women, dates from the early 1800s.

有点像今天的“剩女”的意思。

The idiom “on the shelf” is a common
expression used to describe something that has been abandoned or
forgotten. It is often used in reference to people who are no longer
considered eligible for marriage, or products that have not sold well
and are left unsold on store shelves.

The origins of this idiom can be traced back to the 17th century when
books were stored on shelves in libraries. Books that were not popular
or frequently requested would be placed on higher shelves, where they
would remain untouched and eventually forgotten. Over time, this phrase
became associated with anything that was neglected or overlooked.

During the Victorian era, women who remained unmarried past a certain age were also referred to as being “on the shelf.”
This term was used to suggest that these women had passed their prime
and were no longer desirable as potential wives. The idea behind this
usage of the phrase was rooted in societal expectations about gender
roles and marriage during this time period.

Today, the idiom “on the shelf” continues to be used in various contexts, including business and
marketing. Products that do not sell well may be described as being “on
the shelf,” while employees who are not given opportunities for
advancement may feel like they too have been left behind.

需要注意的是，off-the-shelf 这个习语体现了英语语言的一种思维习惯：动态思维习惯，即把一连串的发生的事，省略一部分叙述，当成理所应当的一个概念，用一个残缺的动态事件来描述一个概念，而不是一个完整的、准确的描述，这与汉语是不同的。

比如汉语里面“迅雷不及掩耳”，英语可能会简化成“迅-雷”，这个会有比较大的歧义。