1. Not easy to believe
   "behind you the coastal hills plunge to the incredibly blue sea backed by the Turkish mountains"
   - improbably, implausibly, unbelievably 
2. Exceedingly; extremely
   - fabulously, fantastically

See also: incredible

Antonym: credibly

• sideshow
• sideslip
• sidesman
• sidesplitter
• sidesplitting
• sidesplittingly
• sidestep
• sidestroke
• sideswipe
• sidetrack
• sidewalk
• sidewall
• sideward
• sidewards
• sideway
• sideways
• sidewinder
• sidewise
• siding
• sidle
• sidle up
• Sidney
• Sidney Caesar
• Sidney James Webb
• Sidney Poitier