Tyrannic, Tyrannical, a.
Etymology: [L. tyrannicus, Gr. tyrannique. See Tyrant.]
Of or pertaining to a tyrant; suiting a tyrant; unjustly severe in government; absolute; imperious; despotic; cruel; arbitrary; as, a tyrannical prince; a tyrannical master; tyrannical government. "A power tyrannical." Shak. Our sects a more tyrannic power assume. Roscommon. The oppressor ruled tyrannic where he durst. Pope. -- Ty*ran"nic*al*ly, adv. -- Ty*ran"nic*al*ness, .
Shades of meaning
Fernald has no entry of its own for tyrannical; he lists it under absolute.
Where he writes about it:
Despotic is commonly applied to a masterful or severe use of power, which is expressed more decidedly by tyrannical. — from absolute
Irresponsible power is not necessarily bad, but eminently dangerous; an executor or trustee should not be irresponsible; an irresponsible ruler is likely to be tyrannical. — from absolute
A perfect ruler might be irresponsible and not tyrannical. — from absolute