含义：
年纪大
溯源：
马匹进入暮年后，咀嚼功能退化，标志便是牙龈萎缩，牙根外露，显得牙齿很长。人们根据马牙的长短来判断其年龄和价值，因此“long in the tooth”引申为“年纪大”。
示例：
She's a bit long in the tooth for such a romantic role.
这样浪漫的角色，她演起来有点太老了
拓展：
成语：“马齿徒增”
比喻空有年岁增长，而学业或事业毫无长进。英语可译为“to be long in the tooth with nothing accomplished”。
其它与“tooth”相关的表达：
fight tooth and nail
解释：竭尽全力地争取某物。
例句：We fought tooth and nail to get the route of the new road changed.
例句：We fought tooth and nail to get the route of the new road changed.
get your teeth into sth
解释：专注于、深入参与某事。
例句：I'm so bored at work, I wish they'd give me something I could really get my teeth into.
例句：I'm so bored at work, I wish they'd give me something I could really get my teeth into.
in the teeth of sth
解释：不顾困难、违抗阻力去做某事。
例句：The road was built in the teeth of fierce opposition from the public.
例句：The road was built in the teeth of fierce opposition from the public.
set sb's teeth on edge
解释：使某人极度恼火（尤指噪音）。
例句：That DJ's voice really sets my teeth on edge.
例句：That DJ's voice really sets my teeth on edge.
希望这些小知识点能帮助你理解“long in the tooth”的含义和用法~