Cause to begin the operation, flow, or activity of, as in Turn on the lights, please, or Don't turn on the sprinkler yet. [First half of 1800s]
Begin to display, employ, or exude, as in He turned on the charm. [Late 1800s]
Also, get high or on. Take or cause to take a mind-altering drug, as in The boys were excited about turning on, or They tried to get her high, or I told them I wouldn't get on tonight. [Slang; mid-1900s]
Be or cause to become excited or interested, as in His mother was the first to turn him on to classical music. [c. 1900]
Be or become sexually aroused, as in He blushed when she asked him what turned him on. [Second half of 1900s]
Also, turn upon. Depend on, relate to, as in The entire plot turns on mistaken identity. This usage, first recorded in 1661, uses turn in the sense of "revolve on an axis or hinge."
Also, turn upon. Attack, become hostile toward, as in Although normally friendly, the dog suddenly turned on everyone who came to the door. Also see turn against.
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