无理由，无缘无故，无根据

Gratuitous \Gra*tu"i*tous\a. [L. gratuitus, from gratus
pleasing. See {Grate}, a., {Gratis.}]
1. Given without an equivalent or recompense; conferred
without valuable consideration; granted without pay, or
without claim or merit; not required by justice.

We mistake the gratuitous blessings of Heaven for
the fruits of our own industry. --L'Estrange.

2. Not called for by the circumstances; without reason,
cause, or proof; adopted or asserted without any good
ground; as, a gratuitous assumption.

Acts of gratuitous self-humiliation. --De Quincye.
-- {Gra*tu"i*tous*ly}, adv. -- {Gra*tu"i*tous*ness}, n.