According to Maneka Gandhi's book of Sanskrit names, the name Kushal means: skilled, efficient, right, good, happy, clever, cool, auspicious, proficient, skill, strategy, efficiency. It is also an epithet Shiva.
There is a folk etymology for the word dating back to ancient India when formal education to Shishya (students) was given by Guru (teacher) in a school called Gurukul. The students helped the guru in his day-to-day life, including carrying out mundane tasks such as washing and cooking. The leaves of a special herb called Kush were required by the gurus as part of their daily rituals. The leaves were very difficult to pluck as they were needle-like, requiring expertise and skill to efficiently remove them from the herb. The most adept students were trained to do the task and once becoming skillful they were given the name Kushal. With the passage of time any expert in any particular field was called Kushal.