Reduction in account equity from a trade or series of trades.  A temporary reduction or decrease in a an upward trend.  Once you accept the concept that all energy, seasons, business, moves in cycles, you can see the drawdown as the trough between one peak and the next. A drawdown is usually quoted as the percentage between the peak and the trough. 
For example the data going back to 1994 shows a cycle pattern, with five major drawdown periods, five recovery periods, and four run-up periods. The average time between major drawdown lows was about 3.5 years, two years of nice returns followed by a year and a half of flat to down. 
c.1900 from draw + down of wells.