"unusual," late 14c., "thin, airy, porous;" mid-15c., "few in number and widely separated, sparsely distributed, seldom found;" from Old French rere "sparse" (14c.), from Latin rarus "thinly sown, having a loose texture; not thick; having intervals between, full of empty spaces," from PIE *ra-ro-, from root *ere- "to separate; adjoin" (source also of Sanskrit rte "besides, except," viralah "distant, tight, rare;" Old Church Slavonic rediku "rare," Old Hittite arhaš "border," Lithuanian irti "to be dissolved"). "Few in number," hence, "unusual." Related: Rareness. In chemistry, rare earth is from 1818.
"undercooked," 1650s, variant of Middle English rere, from Old English hrere "lightly cooked," probably related to hreran "to stir, move, shake, agitate," from Proto-Germanic *hrorjan (source also of Old Frisian hrera "to stir, move," Old Saxon hrorian, Dutch roeren, German rühren, Old Norse hroera), from PIE root *kere- "to mix, confuse; cook" (source also of Greek kera- "to mix," krasis "mixture"). Originally of eggs, not recorded in reference to meat until 1784, and according to OED, in this sense "formerly often regarded as an Americanism, although it was current in many English dialects ...."
"rise up," 1833, dialectal variant of rear (v.1). Sense of "eager" (in raring to go) first recorded 1909. Related: Rared; raring.