This is the first idiom I learnt at university in my English class. It appeared in a text about how children acquire their mother tongue, and for some reason, it's one of those idioms I've never forgotten.
Being lost for words means that you're so shocked, surprised or upset that you don't know what to say. It can have both positive and negative connotations, and the equivalent in Spanish would be quedarse sin palabras or sin habla.
So, as I was saying, this idiom has managed to stay in my mind somehow. I wonder if there's a connection with the fact that, in spite of my age, sometimes life and people still make me lost for words every now and then.