The word "stiffs" is considered to be informal and potentially offensive in some situations. Synonyms for this word include deceased, departed, corpse, cadaver, remains, body, and mortality.
Other options that are less morbid but still convey the same meaning include dullard, unresponsive, unyielding, and rigid. These synonyms are especially useful when referring to people who are inflexible, stubborn, or uncooperative.
In the context of criminal activity, "stiffs" can refer to informants, witnesses, or potential victims who are no longer alive. Synonyms for this definition include mole, rat, snitch, and informer.
In any situation, it is important to choose the appropriate word to avoid offending or upsetting others.