DD 6 does this and had always done this. Her use of words is curious to say the least, although always correct in terms of usage. Just not what and average child or adult for that manner might use, however I have been known to do the same thing and have learned to chose my words based on who I am talking to, although, I have been asked by a friend who is a teacher what a word meant before, apparently I still have an extensive vocabulary LOL. DD had to define lethargic to a teenager she was speaking with the other day. DD loves to play with words so I chalk it up to that.
