a drug on the market指的是滞销商品、滞销货、供应过剩的商品。 drug的本义是“麻醉药品”，而麻醉药品是不能在市场上公开出售的，因此该词常常用来指“滞销货”。如：

The things my son bought home were usually a drug on the market for adults.

我儿子买回来的东西在咱们眼里都是卖不出去的。

这是一个习语。柯林斯字典给出的解释是“a commodity available in excess of the demands of the market”，即市场上供给太多，因而泛滥的商品。

示例：Twenty years ago, radios were a drug on the market. 

20年前，收音机在市场上泛滥成灾。

参考英文释义：The usual meaning of the idiom a drug on the market (also often as a drug in the market) is that it’s a commodity no longer in demand and so is commercially valueless or unsaleable，可翻译为：“卖不出去的商品”、“滞销品”。