乏 is the reversed writing of 正, and the original meaning of 乏 is: not straight, skewed. 乏 serves as a phonetic component to form characters.
《春秋傳》曰：“反正爲乏。”
See also the component 正
In Modern Chinese, 乏 means: lack, useless, tired, poor, exhausted
Characters formed by the semantic component 乏
眨、泛、砭、柉 zhǎ, fàn, biān, fán
姂、疺 fá, fá
贬、窆 biǎn, biǎn