Filled with carelessness with 〜だらけ

Grammar Explanation: Covered in with 〜だらけ

Realizing Urushi is hiding in a box to try and scare him, Ayumu sits with his back to the box.

歩(あゆむ):: 「どうですか？油断(ゆだん)だらけのこの背中(せなか)…」; "How is it? This back filled with carelessness."