1. Immune to attack; incapable of being tampered with
   - impregnable, inviolable, secure, strong, unassailable

See also: invulnerable

• unbendable
• unbending
• unbeneficed
• unbent
• unbiased
• unbiassed
• unbidden
• unbigoted
• unbind
• unbitter
• unbleached
• unblemished
• unblended
• unblessed
• unblinking
• unblinkingly
• unblock
• unbloody
• unblushing
• unblushingly
• unbodied
• unbolt
• unbolted
• unbooked
• unbordered