Old English prættig (West Saxon), pretti (Kentish), *prettig (Mercian) "cunning, skillful, artful, wily, astute," from prætt, *prett "a trick, wile, craft," from West Germanic *pratt- (cf. Old Norse prettr "a trick," prettugr "tricky;" Frisian pret, Middle Dutch perte, Dutch pret "trick, joke," Dutch prettig "sportive, funny," Flemish pertig "brisk, clever"), of unknown origin.
Connection between Old English and Middle English words is uncertain, but if they are the same, meaning had shifted by c.1400 to "manly, gallant," and later moved via "attractive, skillfully made," to "fine," to "beautiful in a slight way" (mid-15c.). Ironical use from 1530s. For sense evolution, compare nice, silly. Also used of bees (c.1400). "After the OE. period the word is unknown till the 15th c., when it becomes all at once frequent in various senses, none identical with the OE., though derivable from it" [OED].
Meaning "not a few, considerable" is from late 15c. With a sense of "moderately," qualifying adjectives and adverbs, since 1560s. Pretty please as an emphatic plea is attested from 1902. A pretty penny "lot of money" is first recorded 1768.