prophylacticyoudaoicibaDictYouDict[prophylactic 词源字典]
prophylactic: [16] Prophylactic comes from Greek prophulaktikós, a derivative of the verb prophulássein. This meant literally ‘keep guard in front of a place’, and hence ‘take precautions against’. It was formed from the prefix pro- ‘before’ and phulássein ‘guard’.
[prophylactic etymology, prophylactic origin, 英语词源]
prophylactic (adj.)youdaoicibaDictYouDict
1570s, originally of medicines, "that tends to prevent disease," from Middle French prophylactique (16c.) and directly as a Latinized borrowing of Greek prophylaktikos "precautionary," from prophylassein "keep guard before, ward off, be on one's guard," from pro- "before" (see pro-) + phylassein, Ionic variant of phylattein "to watch over, to guard," but also "cherish, keep, remain in, preserve" (see phylactery).

The noun is first recorded 1640s, "a medicine or treatment to prevent disease;" meaning "condom" is from 1943, replacing earlier preventive (1822), preventative (1901). Condoms originally were used more to thwart contagious disease than to prevent pregnancy.