WordNet records no synonym and no antonym for steel trap. 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 acute intelligence (an analogy based on the well-known sharpness of steel traps)
“he's as sharp as a steel trap”
a trap made of steel with a strong spring and sharp toothlike projections to hold the prey
Synonyms, antonyms and the broader and narrower relations for steel trap come from Princeton University’s WordNet. Full sources and licences.