Ultraloud Titanium Emergency Whistle
One more reason not to whistle at a lady: she might whistle back at you. At 120 decibels like a mother f'ing banshee. NiteCore's NWS10 claims to be the loudest emergency whistle on the market. Based on the above YouTube reviewer's testing of it--actual demo starts at around 3:45--I would give NiteCore 2 thumbs up for honesty in advertising.
This emergency/survival whistle is made of titanium alloy for combined durability and lightness (0.17 ounces). It is compact enough to attach to a keychain or hang around the neck, with the latter recommended for ladies with racks as spectacular as the one on the lady in NiteCore's press photo.
The whistle is approved as biocompatible and nontoxic to humans, and is corrosion-resistant. It works in wet or dry, hot or cold conditions.