intransitive. To introduce a bitter taste; to be a source of irritation.

intransitive for reflexive.

transitive. colloquial. To prolong or intensify the effect of (something, esp. a painful or unpleasant experience). to pile it on (originally U.S.)…

Proverbial and allusive uses. to rub salt in one's wounds: to behave or speak to someone so as to aggravate a hurt already inflicted.