na-adjective, noun
preposterous; absurd; outrageous
Almost exclusively used in the negative phrase 滅相もない (or めっそうもない), meaning 'don't be ridiculous' or 'that's out of the question'. The standalone adjective use is rare.
See also: 滅相もない (めっそうもない)
「私が責任を取ります」「滅相もない。これは私のミスです」
"I'll take responsibility." "Don't be ridiculous. This is my mistake."
そんな滅相な話は聞いたことがない。
I've never heard such an absurd story.
とんでもない is more common and versatile for 'unthinkable' or 'outrageous', while 滅相 is mostly limited to the set phrase 滅相もない.
The kanji 滅 (destroy) and 相 (aspect) combine to suggest something so extreme it is beyond reason. The exact historical derivation is uncertain, but the word has long been used in the negative phrase 滅相もない.