Interesting how words change. Prophylactic, adjectival form of prophylaxis 'prevention of a disease', a euphemism for a condom, back in the day when they could not be sold as a contraceptive, but as a method of preventing the spread of venereal disease (also a euphemism for sexually transmitted diseases (or STD). From Greek προφυλαξ (prophulaksis) 'advance guard' (cf. French avantgarde), from pro- + phulaks 'guard' (cf. phulassein 'to keep guard before a place / house; (med.) to take precaution against'); it is not in Pokorny IEW, and its origin is summarized by Chantraine as inexplicable and obscure. (But I'm sure that hasn't stopped any étymologistes manqués from connecting it to Sumerian, Basque, Etruscan, High Martian, and Proto-Lemurian.

Ceci n'est pas un seing.