1. immune to attack; incapable of being tampered with

- an impregnable fortress

Similar word(s): invulnerable, inviolable, secure, strong, unassailable, unattackable

2. capable of conceiving

Similar word(s): fertile, conceptive

3. incapable of being overcome, challenged or refuted

- an impregnable argument

- impregnable self-confidence

Similar word(s): unconquerable, inexpugnable

Sentences with impregnable as an adjective:

- Following treatment, twenty percent of previously infertile females were impregnable.Application of the compound rendered the non-porous surface impregnable.