半吊子

Etymology
In ancient China, a diao was a thousand copper coins strung together. Half a diao (半吊) referred to five hundred coins, and became to be used metaphorically for someone who is deficient in skills or mental capacity. Compare.

Noun

 * 1)  one who lacks adequate knowledge or skills; dilettante; smatterer; dabbler
 * 2)  hot-headed or impulsive person; one who leaves tasks unfinished, or the tasks which are unfinished