1. Immune to attack; incapable of being tampered with
   "an impregnable fortress"
   - inviolable, secure, strong, unassailable, unattackable 
2. Capable of conceiving
   - conceptive 
3. Incapable of being overcome, challenged or refuted
   "an impregnable argument"; "impregnable self-confidence"
   - inexpugnable

See also: fertile, impregnability, impregnably, invulnerable, unconquerable

Encyclopedia: Impregnable

• knitting
• knitting machine
• knitting needle
• knitting stitch
• knitwear
• knitwork
• knob
• knob celery
• knobbed
• knobble
• knobbly
• knobby
• knobcone pine
• knobkerrie
• knobkerry
• knock
• knock against
• knock back
• knock cold
• knock on
• knock over
• knock rummy
• knock-down-and-drag-out
• knock-knee
• knock-kneed