被
bèi · HSK 3
by
被 = 衤(clothes) + 皮(skin/cover); a blanket covers you, hence passive 'by'
Originally meant blanket/quilt — something that covers the sleeper. The 'covered by' sense extended to grammatical passive marker: the subject is acted on 'by' someone, just as a body is covered by a quilt.
被 — clothes radical 衤 + 皮 (skin/hide)
Originally a semantic-phonetic compound: 衤 (clothing) gives the meaning 'blanket/quilt', while 皮 supplies the sound. From 'covering' it extended to 'cover oneself with', then grammaticalized into the passive marker 'by', its most common modern use.