Explanation:

- Option 1: Habituated to using difficult words

This means someone is accustomed to using complex vocabulary regularly.

- Option 2: Not aware of the language

This implies a lack of knowledge or understanding of a particular language.

- Option 3: Not know what to say

This idiom means being speechless or unable to find the right words in a given situation.

This is the correct interpretation of the idiom "Be at a loss for words."

- Option 4: Lost the urge to speak

This suggests a lack of desire to speak but doesn't correctly capture the idiom's meaning.