My wife asks interesting questions about words, and the most recent was “Why do we say ‘bedridden’?” I opened my mouth, realized I didn’t have anything useful to say, and turned to the dictionary. The answer is simple but unpredictable, and since others may well be interested, I’m sharing it here. The Online Etymology Dictionary has a good summary:
also bed-ridden, mid-14c., from adjectival use of late Old English bæddrædæn “bedridden (man),” from bedrid, from Old English bedreda, literally “bedrider, bedridden (man),” from bed + rida “rider” (see ride (v.)). Originally a noun, it became an adjective in Middle English and acquired an –en on the analogy of past participle adjectives from strong verbs such as ride.
So it was originally ‘bed-rider,’ which makes sense, and due to the sort of morphological scrambling languages are subject to, it looks like it means ‘ridden by a bed,’ which doesn’t.