WordNet records no synonym and no antonym for cheap shot. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
an unnecessarily aggressive and unfair remark directed at a defenseless person
an illegal and unsportsmanlike act of unnecessary violence
“he called a penalty on them when the lineman took a cheap shot at the quarterback”
Synonyms, antonyms and the broader and narrower relations for cheap shot come from Princeton University’s WordNet. Full sources and licences.