When the word "kamikaze" was first used, what did it mean?

The original meaning of the word "kamikaze" had nothing to do with Japanese suicidal missions during war. The word was initially translated as "divine wind". It acquired its meaning, unconventional for most people, after a typhoon, called the same name, had twice disrupted Mongolian Army's attacks on Japan.

In history, the Kamikaze attacks of 1274 and 1281 were a pair of massive typhoons (tropical cyclones) that each wrecked a Mongol fleet attempting to invade Japan in 1274 and 1281. The storms destroyed most of the Mongol ships and dispersed the rest. The Mongol attackers were forced to abandon their plans; Japan was very fortuitously saved from both foreign attempts at conquest.

This term (kamikaze) was later used in World War II to refer to the Japanese suicide pilots who deliberately crashed their planes into targets of Allied forces, usually ships.