What is slack key guitar?

Slack key guitar is a style of finger-style guitar played in open tuning that originated in 19th century Hawaii. The style was created when Portuguese cowboys brought the guitar to Hawaii. The Hawaiians chose to retune the guitar by slackening some of the strings often to an open G chord. The most popular tuning DGDGBD is called "taro patch". Families often had their own tunings which were held as closely guarded secrets.