IPA: /ˈbɛdˌfæst/
KK: /bɛdˌfæst/
Unable to leave bed due to illness or disability.
After the surgery, she was bedfast for several weeks.
Bedfast is formed from "bed" (from Old English "bed", meaning a place for sleeping) and "fast" (from Old English "fæst", meaning firmly or fixed). The word "bedfast" describes someone who is confined to bed, unable to get up, thus being firmly in bed.
Think of someone who is 'fixed' ('fast') to their 'bed' — that's why bedfast means unable to leave the bed.