| hanging tough(expression, adjective, persistent, constant, pitiless) |
| hard-headed(adjective, persistent, constant, pitiless, unyielding) |
| hourly |
| illimitable(adjective, constant, abundant, unrestrained, endless) |
| immeasurable(constant, abundant, unrestrained, endless, immense) |
| immediate |
| immemorial |
| imperishable(stable, constant, continuous, enduring, lasting) |
| in for long haul |
| in for the long haul(expression, idiom, stable, enduring, lasting) |
| inborn(fixed, persistent, continuous, lasting, continuing) |
| inbred(fixed, persistent, continuous, lasting, continuing) |
| incalculable(constant, abundant, unrestrained, endless, immense) |
| incurable(fixed, persistent, continuous, lasting, continuing) |
| indelible(permanent, long-term, lifelong) |
| indeterminate(constant, abundant, monotonous, unrestrained, endless) |
| indissoluble(permanent, long-term, lifelong) |
| ineradicable(fixed, persistent, continuous, lasting, continuing) |
| inexhaustible(certain, reliable, trustworthy, permanent, boundless) |
| inexpungible(permanent, long-term, lifelong) |