It literally means "a foot and a half long," but it's used figuratively to describe a person who uses long and often unnecessarily complex words.
Here are some other related words:
* bombastic: using inflated language to impress
* grandiloquent: using pompous or bombastic language
* pretentious: attempting to impress by affecting greater importance, talent, culture, etc., than is actually possessed
* verbose: using more words than necessary