*rei-

late 14c., "descend from," from Old French deriver "to flow, pour out; derive, originate," from Latin derivare "to lead or draw off (a stream of water) from its source" (in Late Latin also "to derive"), from phrase de rivo (de "from" + rivus "stream," from PIE root *rei- "to run, flow").

From c. 1500 as "obtain by a process of reasoning." In reference to words, "arise by a process of word-formation," 1550s; meaning "trace or show derivation" is from c. 1600. General sense of "get, gain, obtain" (as from a source or origin) is from 1560s; that of "arise, spring" (from) a source or origin is from 1660s. Related: Derived; deriving.

Old English Ymbrendaeg, Ymbren, 12 days of the year (divided into four seasonal periods, hence Medieval Latin name quatuor tempora) set aside by the Church for fasting and prayers, from Old English ymbren "recurring," corruption of ymbryne "a circuit, revolution, course, anniversary," literally "a running around," from ymb "round" (from Proto-Germanic umbi, from PIE root *ambhi- "around") + ryne "course, running" (from PIE root *rei- "to run, flow"). Perhaps influenced by a corruption of the Latin name (compare German quatember, Danish tamper-dage). The Wednesday, Friday, and Saturday after the first Sunday in Lent, Whit-Sunday, Sept. 14, and Dec. 13, set aside for prayer and fasting.