Putting a bridle on a horse; or metaphorically, controlling, restraining, or checking something.
From 'bridle' (Old English 'bridel') + '-ing' gerund suffix. The metaphorical sense developed early as restraint became the central concept.
The phrase 'bridling at' something (as in 'bridling at insult') shows how this word moved from equestrian vocabulary into emotional territory—a horse that bridles at the bit resists control, just as humans bridle at what they resist.
Complete word intelligence in one call. Free tier — 50 lookups/day.