1. Not easy to believe
   - incredibly, improbably, implausibly 
2. In an unbelievable manner
   "he was unbelievably angry"

See also: unbelievable

Antonym: believably

• substitution
• substitution class
• substitutional
• substitutionary
• substitutive
• substract
• substrate
• substratum
• substring
• substructure
• subsume
• subsumption
• subsurface
• subsystem
• subtenant
• subtend
• subterfuge
• subterminal
• subterranean
• subterraneous
• subtext
• subthalamic nucleus
• subthalamus
• subtile
• subtilin