Old English god (with a long "o") "virtuous; desirable; valid; considerable," probably originally "having the right or desirable quality," from Proto-Germanic *gothaz (cf. Old Norse goðr, Dutch goed, Old High German guot, German gut, Gothic goþs), originally "fit, adequate, belonging together," from PIE root *ghedh- "to unite, be associated, suitable" (cf. Old Church Slavonic godu "pleasing time," Russian godnyi "fit, suitable," Old English gædrian "to gather, to take up together"). As an expression of satisfaction, from early 15c.; of children, "well-behaved," by 1690s.
Irregular comparatives (better, best) reflect a widespread pattern, cf. Latin bonus, melior, optimus. Good-for-nothing is from 1711. Good looking is attested from 1780 (good looks by c.1800). Good sport, of persons, is from 1906; good to go is attested from 1989. The good book "the Bible" attested from 1801, originally in missionary literature describing the language of conversion efforts in American Indian tribes.