prefix
Prefix attached to nouns or adjectives to express a weak, faint, or slight degree. Often used in words like うすら寒い (chilly) or うすら明かり (dim light).
朝方、うすら寒さを感じて目が覚めた。
I woke up in the early morning feeling a slight chill.
部屋にはうすら明かりが差し込んでいた。
A dim light was coming into the room.
薄い is an adjective meaning 'thin' or 'pale', while うすら is a prefix that adds a sense of 'slight' or 'faint' to the following word.
Derived from the stem of the adjective 薄い (usui, 'thin, pale'), with the suffix ら, which can indicate a vague state or manner. The exact historical development is uncertain.