If you don't know the etymology for carabiner, make a quick guess before you read on. Go on, it's fun!
The word carabiner first emerges from the mists of time as Calabria -- yes, that's right, the city in the south of Italy. The exact origins of the name Calabira are uncertain, but Cala, at the time, was in use to refer to a protected place, such as a bay, and 'bria' was the Thracian word for "city". But 'Bay City' is a long way from 'metal D-ring', so let's keep going...
In the late 1500s, Calabria became known for its light cavalry, horsemen who were armed with a short, light-weight rifle. These Calabrians faded from history, but the shortened rifle proved quite useful and remained, known by the now cryptic name 'carabin', later to wend its way into English as 'carbine'. Carbines were very popular for use with the light cavalry for centuries, and were used by Carabiniers in the Napoleonic wars and later the Dragoons. In World War I the German troops were issued Karabiners, but more than this they were issued Karabinerhaken, D-shaped hooks to fasten the carbines to their bandoleers.
The Germans, being efficient people, realized that the hook was the more useful part of the gear, and soon Karabiner came to be the popular name for the hook. The English, unwilling to leave a foreign word alone, changed the spelling to 'carabiner', and thus it stands today.
But perhaps not for long... Climbers are starting to use the shortened 'biner' to refer to carabiners, and it is only a matter of decades before they become known as binders, and yet another word is completely absorbed into the Anglo-Saxon etymosphere.