The decoration or finishing of the top edge of a boot, or the nautical practice of coating a ship's hull near the waterline to prevent damage. In maritime use, it's a protective paint treatment.
From 'boottop' + the gerund suffix '-ing' (Old English '-ing'). The nautical meaning emerged from the visual similarity between a ship's waterline stripe and a boot's top edge.
Naval ships have 'boottops'—painted stripes at the waterline—and sailors historically painted them in different colors to show which country or navy owned the ship! It's both practical (protecting the hull from water damage) and artistic.
Complete word intelligence in one call. Free tier — 50 lookups/day.