1 weak, feeble, frail, rickety, shaky, ramshackle, infirm, unstable, wobbly, tottering, unsteady, broken-down, crumbling, disintegrating, dilapidated, defective, imperfect, faulty, decayed, rotten:
The building inspector condemned the structure as unsound and unsafe.
2 unhealthy, diseased, ill, afflicted, in poor health, ailing, sickly, sick, unwell, delicate, injured, wounded:
The attack of flu left him unsound in wind and limb.
3 insane, mad, psychotic, unbalanced, unstable, demented, deranged:
The doctors said that she was still of unsound mind and ought not be released.
4 illogical, faulty, flawed, fallacious, untenable, invalid, groundless, unfounded, erroneous, defective, specious:
We thought that his arguments were unsound, so we rejected his recommendations.