Long：长的，tooth：牙齿；字面来看呢，long in the tooth是牙齿长的意思；
In english：Old in age. Mostly used when referring to people or horses.英文里指的是年纪大，一般指的是人或者一匹马；
This phrase originates from horses, or more specifically, it comes from one’s ability to discern a horse's age simply by looking at their teeth. 这个俚语与马有关。有的人只是看下马的牙齿，就可以判断出马的年龄。The longer the teeth, the older the horse. 牙齿长，年纪大。Thus, this phrase is used to describe how someone or something is long in years.所以这个俚语就用来被描述一个年纪大的人或者事物。While the phrase apparently began with horses, it eventually was applied to describe the age in humans too, among other things.虽然这个词刚开始用来描述马，但是最终也用来指代人或者其他事物；
The desktop was getting long in the tooth.
Don't you think you're a little long in the tooth for it?