"smoked herring" early 15c. (they turn red when cured), as opposed to white herring "fresh herring." Supposedly used by fugitives to put bloodhounds off their scent (1680s), hence metaphoric sense (1864) of "something used to divert attention from the basic issue;" earlier it simply meant "a false lead":
Though I have not the honour of being one of those sagacious country gentlemen, who have so long vociferated for the American war, who have so long run on the red-herring scent of American taxation before they found out there was no game on foot; (etc.) [Parliamentary speech dated March 20, 1782, reprinted in "Beauties of the British Senate," London, 1786]
In the sense of "person liberally and excessively sympathetic" (especially toward those the speaker or writer deems not to deserve it) is attested by 1951, but said by many to have been popularized with reference to liberals (especially Eleanor Roosevelt) in 1930s by newspaper columnist Westbrook Pegler (1894-1969), though quotations are wanting; bleeding in a figurative sense of "generous" is from late 16c., and the notion of one's heart bleeding as a figure of emotional anguish is from late 14c.; the exact image here may be the "bleeding heart of Jesus."
name borne by two rulers of Tenochtitlan in ancient Mesoamerica, from Spanish Moctezuma, from Nahuatl (Aztecan) Moteuczoma, said to mean "he who frowns like a lord" or "he who is angry in a noble manner." Montezuma's revenge, "severe intestinal infection" sometimes suffered by non-natives in Mexico, is by 1962, in reference to Montezuma II (1466-1520), Aztec ruler at the time of the Spanish arrival and conquest of Mexico.