c. 1300, "an equal in rank, character, or status" (early 13c. in Anglo-Latin), from Anglo-French peir, Old French per (10c.), from Latin par "equal" (see par (n.)). Sense of "a nobleman of especial dignity" (late 14c.) is from Charlemagne's Twelve Peers in the old romances, who, like the Arthurian knights of the Round Table, originally were so called because all were equal. Sociological sense of "one of the same age group or social set" is from 1944. Peer review "evaluation of a scientific project by experts in the relevant field" is attested by 1970. Peer pressure is recorded by 1971.
"to look closely," 1590s, variant of piren (late 14c.), with a long -i-, probably related to or from East Frisian piren "to look," of uncertain origin. Influenced in form and sense by Middle English peren (late 14c.), shortened form of aperen (see appear). Related: Peered; peering.
1690s, originally an art criticism term, "assemblage of figures or objects forming a harmonious whole in a painting or design," from French groupe "cluster, group" (17c.), from Italian gruppo "group, knot," which probably is, with Spanish grupo, from a Germanic source, from Proto-Germanic *kruppaz "round mass, lump," part of the general group of Germanic kr- words with the sense "rounded mass" (such as crop (n.).
Extended to "any assemblage, a number of individuals related in some way" by 1736. Meaning "pop music combo" is from 1958. As it was borrowed after the Great Vowel Shift in English, the pronunciation of the -ou- follows French rather than English models.