Soaking wet; completely drenched (clothes, hair, etc.).
帰り道で急に雨に降られて、靴までびしょ濡れになった。
On the way home I got caught in sudden rain and my shoes were soaked.
子どもたちは水たまりで遊んでびしょ濡れだ。
The children played in the puddles and are dripping wet.
傘を忘れて会社に着いたときには上着がびしょ濡れだった。
I forgot my umbrella and when I arrived at the office my coat was soaked.