Refers to a state of being weaker or inferior in power, influence, or position. Often used in contrast to 強勢 (きょうせい, 'strength' or 'dominance').
We were placed in a position of weakness in the negotiations.
There are many situations where one feels inferiority.
Compound of 弱 (じゃく, 'weak') and 勢 (せい, 'power' or 'force'). The exact historical derivation is uncertain, but the meaning is transparent from its components.