it is always a mortal sin.
But this is apparently untrue, as may be seen in the case of one who
utters a reviling word indeliberately or through slight anger.
Therefore reviling or railing is not a mortal sin, by reason of its
genus.

_On the contrary,_ Nothing but mortal sin deserves the eternal
punishment of hell. Now railing or reviling deserves the punishment
of hell, according to Matt. 5:22, "Whosoever shall say to his brother
. . . Thou fool, shall be in danger of hell fire." Therefore railing
or reviling is a mortal sin.

_I answer that,_ As stated above (A. 1), words are injurious to other
persons, not as sounds, but as signs, and this signification depends
on the speaker's inward intention. Hence, in sins of word, it seems
that we ought to consider with what intention the words are uttered.
Since then railing or reviling essentially denotes a dishonoring, if
the intention of the utterer is to dishonor the other man, this is
properly and essentially to give utterance to railing or reviling:
and this is a mortal sin no less than theft or robbery, since a man
loves his honor no less than his possessions. If, on the other hand,
a man says to another a railing or reviling word, yet with the
intention, not of dishonoring him, but rather perhaps of correcting
him or with some like purpose, he utters a railing or reviling not
formally and essentially, but accidentally and materially, in so far
to wit as he says that which might be a railing or reviling. Hence
this may be sometimes a venial sin, and sometimes without any sin at
all. Nevertheless there is need of discretion in such matters, and
one should use such words with moderation, because the railing might
be so grave that being uttered inconsiderately it might dishonor the
person against whom it is uttered. In such a case a man might commit
a mortal sin, even though he did not intend to dishonor the other
man: just as were a man incautiously to injure grievously another by
striking him in fun, he would not be without blame.

Reply Obj. 1: It belongs to wittiness to utter some slight mockery,
not with intent to dishonor or pain the person who is the object of
the mockery, but rather with intent to please and amuse: and this may
be without sin, if the due circumstances be observed. On the other
hand if a man does not shrink from inflicting pain on the object of
his witty mockery, so long as he makes others laugh, this is sinful,
as stated in the passage quoted.