Weakness, Feebleness
noun, な adjective
Something is diluted to the point of weakness. It has much weakness and feebleness.
はくじゃく
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
お兄さんに言われたわ。あなたは意志薄弱だから私と別れることができないけど、本当は別れたいんだって。
Your brother told me that you are too weak-willed to break up with me, but that you really want to.