Old English bisceop "bishop, high priest (Jewish or pagan)," from Late Latin episcopus, from Greek episkopos "watcher, (spiritual) overseer," a title for various government officials, later taken over in a Church sense, from epi- "over" (see epi-) + skopos "one that watches, one that looks after; a guardian, protector" (from PIE root *spek- "to observe"). Given a specific sense in the Church, but the word also was used in the New Testament as a descriptive title for elders, and continues as such in some non-hierarchical Christian sects.
A curious example of word-change, as effected by the genius of different tongues, is furnished by the English bishop and the French évêque. Both are from the same root, furnishing, perhaps the only example of two words from a common stem so modifying themselves in historical times as not to have a letter in common. (Of course many words from a far off Aryan stem are in the same condition.) The English strikes off the initial and terminal syllables, leaving only piscop, which the Saxon preference for the softer labial and hissing sounds modified into bishop. Évêque (formerly evesque) merely softens the p into v and drops the last syllable. [William S. Walsh, "Handy-Book of Literary Curiosities," Philadelphia, J.B. Lippincott, 1892]
Late Latin episcopus in Spanish became obispo, in Italian vescovo, in Welsh esgob. The Germanic forms include Old Saxon biscop, Old High German biscof. Further afield it became Lithuanian vyskupas, Albanian upeshk, Finnish piispa. A once-popular pun on it was bite-sheep (1550s, also in German, biss-schaf). The chess piece (formerly archer, before that alfin) was so called from 1560s.