terrific comes from: < French (now arch.) terrifique causing terror (1496 in Middle French as terriffique ), extraordinary (1507) and its etymon classical Latin terrificus terrifying, frightening, awe-inspiring < terrēre to frighten (see terrible adj.) + -ficus -fic suffix. Compare Spanish terrífico (c1500), Italian terrifico (a1527).
From the OED, it appears to have switched from usage meaning "terrifying" to "splendid" in the 1800s; first usage as "splendid" comes from 1871; last usage as "terrifying" is from 1914.
Etymology of "terrible" is: < Anglo-Norman and Middle French terrible (French terrible ) causing or fit to cause terror (c1160 in Old French), awe-inspiring (second half of the 14th cent.), very harsh, severe, or painful, very great (second half of the 15th cent.; the use in sense A. 2b is apparently not paralleled in French until later: 1672) and its etymon classical Latin terribilis inspiring terror or alarm, frightening < terrēre to frighten (ultimately < the same Indo-European base as Sanskrit tras- to tremble, ancient Greek τρεῖν to flee, to fear, dread) + -bilis -ble suffix. Compare Old Occitan tarrible (15th cent.), Catalan terrible (14th cent.), Spanish terrible (end of the 13th cent.), Portuguese terrível (15th cent. as †terribel , †terribil ), Italian terribile (early 14th cent.). With use as noun compare post-classical Latin terribilia (neuter plural) terrible marvels (Vulgate), French terrible terrible thing (1669). With use as adverb compare slightly earlier terribly adv.
Both "terrible" and "terrific" are defined in the OED as meaning "awe-inspiring".