Lethargic and lethargies comes from the old Greek (or was it Latin, too lazy to check). It's a relatively common loan word in most of the European languages. My

Lethargic and lethargies comes from the old Greek (or was it Latin, too lazy to check). It's a relatively common loan word in most of the European languages. My guess is that for most slightly educated non-native English speakers that word is more likely to be known than its English equivalent if it exists.