Semtex is a brand of plastic explosive that English news has used so often it now works like a type name. It is a soft, putty-like explosive, first made in what is now the Czech Republic. Armies and people who take down old buildings have used plastic explosive for real work. Though actually, in English headlines the word is soaked in the illegal story — bombs, attacks — because that is how many readers first met it. The tidy learner-dictionary line “used illegally to make bombs” is one famous use, not the whole life of the product. You write it with a capital S as a trademark, though papers sometimes drop that. You do not need this word in daily life; it belongs to news, crime reports, and history. Say the noun. There is no reason for a student to practise it as a verb.

Semtex（塑膠炸藥的品牌名；台灣新聞也常直接說「塑膠炸藥」）